Add label to x-axis in tree panel

[trvrb]

Currently the "Phylogeny" panel doesn't have a label for the x-axis:

This lack of labeling is hugely confusing to outsiders and everyone knows this is poor data viz practice. Let's fix this. I recommend:

• "Date" if showing a tree with mu=num_date.
• "Mutations" if showing a tree with m=div and branch lengths are big enough to indicate this this measures substitutions rather than substitutions per site.
• "Divergence" if showing a tree with m=div and branch lengths are small enough to indicate this this measures substitutions per site rather than substitutions.

Logic to test between Substitutions and Substitutions per site is here: 80915fd

I wouldn't worry about this labeling for "radial" or "unrooted" views.

[huddlej]

Closed by #942