ENH: Return distances option for linkage_trees
Merge return_distance tests of both linkage and ward tree
Add documentation for return_distances option
TST: Test ground truth in linkage trees
MAINT remove deprecated auc_score function
Merge pull request #3985 from MechCoder/return_distances

[MRG] ENH: Return distances option for linkage_trees
COSMIT flat > nested / avoid duplicate validation

Also, PEP8