* First cut at basic single linkage internals
* Refer to correct dist_metrics package
* Add csgraph sparse implementation for single linkage
* Add fast labelling/conversion from MST to single linkage tree; remove uneeded single_linkage.pyx file.
* Ensure existing tests cover single linkage
* Iterating toward correct solution. Still have to get n_clusters, compute_full_tree=False working
* Fix the parents array when we don't get the "full tree"
* Add single linkage to agglomerative clustering example.
* Add single linkage to digits agglomerative clustering example.
* Update documentation to reflect the addition of single linkage.
* Update documentation to reflect the addition of single linkage.
* Provide a more complete comparison of the different linkage methods, highlighting the relative strengths and weaknesses.
* Update docs with a more complete comparison on linkage methods (scale to be determined?)
* Update WhatsNew for single linkage clustering.
* Handle true zero distances by setting them to "epsilon" distances
* Add test for identical points messing with sparse linkage clustering.
* Test all the linkage methods for identical point issues
* Cast precomputed distances to float64 for consistency
* Turn bounds checking off; add docsting warning.
* Make public and private versions of labelling.
* more efficient is sorted check
* Explicit cast to cover all bases
Leland McInnes