ward_tree returns heights (distances) between the clusters
both for the structured and unstructured version. Useful for
plotting the dendrogram with scipy.
ENH: `ward_tree` returns distances

Returns distances for both structured and unstructured version,
and they are equal for both versions.

Added test under test_hierarchical.
changed xrange to range for compatibility with Python 3
TST, test ward_tree_distance on known dataset

- Solve PEP8 problems in test_ward_tree_distance and ward_tree
- Refactor return_height into return_distance for ward_tree.
- Test that structured and unstructured versions of ward_tree return the
  same distances on a known dataset.
- structured and unstructured versions return children with the same
  order
FIX: structured and unstructured ward_tree return children sorted in the
same way

Add test
Clean code for test_ward_tree_distance
Cosmetic change in test_hierarchical, better documentation
Add ENH description in whats_new.rst
Add better description of distances, cosmetic changes
Fix git rebase problems
Add forgotten IFs
Clean code
Clean code
Merge pull request #3158 from mvdoc/clustering

[ENH] ward_tree: return distances