Rename tree_height_scale to time_scale?

[jeromekelleher]

I had to look the docs up for this just now - it would be more obvious to have e.g. time_scale="rank" rather than tree_height_scale="rank". Not sure if there's any good reason for what it is currently?

We'd keep tree_height_scale as a deprecated synonym.

@hyanwong, thoughts?

[hyanwong]

time_scale seems fine to me. I can't remember what rationale (if any) there was behind tree_height_scale.