update contributing guidelines to mention creating unit tests and regression test data. add checkbox to pull request template

.github/CONTRIBUTING.md

@@ -75,7 +75,9 @@ Here is a checklist of steps you need to follow to submit a single file or user
 * Your new source files need to have the LAMMPS copyright, GPL notice, and your name and email address at the top, like other user-contributed LAMMPS source files. They need to create a class that is inside the LAMMPS namespace. If the file is for one of the USER packages, including USER-MISC, then we are not as picky about the coding style (see above). I.e. the files do not need to be in the same stylistic format and syntax as other LAMMPS files, though that would be nice for developers as well as users who try to read your code.
 * You **must** also create or extend a documentation file for each new command or style you are adding to LAMMPS. For simplicity and convenience, the documentation of groups of closely related commands or styles may be combined into a single file. This will be one file for a single-file feature. For a package, it might be several files. These are files in the [reStructuredText](https://docutils.sourceforge.io/rst.html) markup language, that are then converted to HTML and PDF. The tools for this conversion are included in the source distribution, and the translation can be as simple as doing "make html pdf" in the doc folder. Thus the documentation source files must be in the same format and style as other `<name>.rst` files in the lammps/doc/src directory for similar commands and styles; use one or more of them as a starting point. An introduction to reStructuredText can be found at [https://docutils.sourceforge.io/docs/user/rst/quickstart.html](https://docutils.sourceforge.io/docs/user/rst/quickstart.html). The text files can include mathematical expressions and symbol in ".. math::" sections or ":math:" expressions or figures (see doc/JPG for examples), or even additional PDF files with further details (see doc/PDF for examples). The doc page should also include literature citations as appropriate; see the bottom of doc/fix_nh.rst for examples and the earlier part of the same file for how to format the cite itself. The "Restrictions" section of the doc page should indicate that your command is only available if LAMMPS is built with the appropriate USER-MISC or USER-FOO package. See other user package doc files for examples of how to do this. The prerequisite for building the HTML format files are Python 3.x and virtualenv. Please run at least `make html`, `make pdf` and `make spelling` and carefully inspect and proofread the resulting HTML format doc page as well as the output produced to the screen. Make sure that all spelling errors are fixed or the necessary false positives are added to the `doc/utils/sphinx-config/false_positives.txt` file.  For new styles, those usually also need to be added to lists on the respective overview pages. This can be checked for also with `make style_check`.
 * For a new package (or even a single command) you should include one or more example scripts demonstrating its use. These should run in no more than a couple minutes, even on a single processor, and not require large data files as input. See directories under examples/USER for examples of input scripts other users provided for their packages. These example inputs are also required for validating memory accesses and testing for memory leaks with valgrind
-* If there is a paper of yours describing your feature (either the algorithm/science behind the feature itself, or its initial usage, or its implementation in LAMMPS), you can add the citation to the *.cpp source file. See src/USER-EFF/atom_vec_electron.cpp for an example. A LaTeX citation is stored in a variable at the top of the file and a single line of code that references the variable is added to the constructor of the class. Whenever a user invokes your feature from their input script, this will cause LAMMPS to output the citation to a log.cite file and prompt the user to examine the file. Note that you should only use this for a paper you or your group authored. E.g. adding a cite in the code for a paper by Nose and Hoover if you write a fix that implements their integrator is not the intended usage. That kind of citation should just be in the doc page you provide.
+* For new utility functions or class (i.e. anything that does not depend on a LAMMPS object), new unit tests should be added to the unittest tree.
+* When adding a new LAMMPS style, a .yaml file with a test configuration and reference data should be added for the styles where a suitable tester program already exists (e.g. pair styles, bond styles, etc.).
+* If there is a paper of yours describing your feature (either the algorithm/science behind the feature itself, or its initial usage, or its implementation in LAMMPS), you can add the citation to the <name>.cpp source file. See src/USER-EFF/atom_vec_electron.cpp for an example. A LaTeX citation is stored in a variable at the top of the file and a single line of code that references the variable is added to the constructor of the class. Whenever a user invokes your feature from their input script, this will cause LAMMPS to output the citation to a log.cite file and prompt the user to examine the file. Note that you should only use this for a paper you or your group authored. E.g. adding a cite in the code for a paper by Nose and Hoover if you write a fix that implements their integrator is not the intended usage. That kind of citation should just be in the doc page you provide.
 
 Finally, as a general rule-of-thumb, the more clear and self-explanatory you make your documentation and README files, and the easier you make it for people to get started, e.g. by providing example scripts, the more likely it is that users will try out your new feature.
 

.github/PULL_REQUEST_TEMPLATE.md

@@ -34,6 +34,7 @@ By submitting this pull request, I agree, that my contribution will be included
 - [ ] The added/updated documentation is integrated and tested with the documentation build system
 - [ ] The feature has been verified to work with the conventional build system
 - [ ] The feature has been verified to work with the CMake based build system
+- [ ] Suitable tests have been added to the unittest tree.
 - [ ] A package specific README file has been included or updated
 - [ ] One or more example input decks are included