I have two motivations for this:

1. Within the context of user code, what we have been calling
   FigureSpec is the complete user-facing description of "the figure".
   Thus, I am in the habit of using the local variable name `figure`
   for instances of FigureSpec. @jklynch pointed out that this is
   confusing. If we just call the call Figure, this inconsistency goes
   away.
2. I want to shorten the names in preparation for potentially adding
   varieties.

- Make Artists aware only on update(), not BlueskyRun details.
- Make Axes know about Artists, not separating specific types.