Nested calls to rc.context() fail

[lukelbd]

You can end up with persistent changes to the rc object if rc.context() statements are nested. Most notably

import proplot as plot
with plot.rc.context(...):
    f, ax = plot.subplots()
    ax.format(...)

fails because ax.format calls rc.context(...) internally. #50 will fix this (I ended up putting some quite tangentially related stuff in that PR... will try to avoid this in the future).

[lukelbd]

Actually rc.context is completely broken... because Axes.__init__() calls self.format(mode=1) to implement the custom proplot rcParamsLong params (e.g. abc, abc.style, land, land.color, etc.).

[lukelbd]

Instead of waiting for #50 to be merged, I'll try to copy the changes in #50 unrelated to add_subplot, GeometrySolver, and EdgeStack into the main branch. That will resolve this issue.

I originally put so much extra stuff in #50 because I thought it would be finished in November; now it looks like it will have to wait until the middle of next year sometime. Learned a hard lesson about dividing up tasks into small PRs.

[lukelbd]

Closed by #91