BUG: Fix dendrogram labeling, per #3822

[jamestwebber]

Fixing a bug (#3822) in which label formatting arguments to scipy.cluster.hierarchy.dendrogram were being ignored. Also, made the behavior of the formatting more consistent across the four orientation options. Unless user-specified, label size will be autoscaled based on the number of labels. For "top" and "bottom" oriented dendrograms, the labels will be rotated if there are a lot of them.

In my own tests this version has the desired behavior. Unfortunately plot outputs are a pain to write tests for, otherwise I would add one in.

[WarrenWeckesser]

I noticed another (older) bug that you might as well fix while you are working on this. The code

if leaf_rotation:
    ... do something ...
else:
    ... do something else ...

will do the wrong thing if the user specifies leaf_rotation=0. The test should be if leaf_rotation is not None. Similarly, if leaf_font_size: should probably be if leaf_font_size is not None:, but I don't know what the point of setting leaf_font_size=0 would be.

[jamestwebber]

Good point, fixed.

[jamestwebber]

For what it's worth I think the failed test was just due to time--the same test passed in an earlier commit and all I changed was indentation.

[rgommers]

@jamestwebber thanks for this PR. I've taken it over and built on top of it in gh-5164, so I'm closing this one.

A review of my PR would be very helpful.