PR Summary

This is an attempt at implementing a nicer API for laying out complex axes schemes like patchwork for ggplot. This is following on from a conversation @anntzer and @story645 had a few weeks ago
https://hackmd.io/4zWKhuLXQ16Y_oUfi9hnKA#Gridspec-Easy-API and was done on a red-eye flight home

The big win here is things like

x = [["A", "A", "B"], ["C", "D", "B"]]
fig = plt.figure()
axes = fig.build_grid(x)
axes['A'].plot(...)

do what you expect.

open questions:

• Name (Hannah suggested something riffing to "collage" which I think is good if we can make it a verb)
• does this go in Matplotib or does this go in a small library under the mpl org?
• better tests of things going wrong
• be sure that when re-assigning grid specs we clean up all the evidence of the old one (to be sure constrained layout works etc)
• do we want the expansion logic to work ([['A'], ['B', 'C']] <-> [['A', 'A'][ ['B', 'C']])
• keys in return dict when adjusting existing axes
• provide a way to have mixed subplot_kw to each of the created figures to do later

PR Checklist

• Has Pytest style unit tests
• Code is Flake 8 compliant
• New features are documented, with examples if plot related
• Documentation is sphinx and numpydoc compliant
• Added an entry to doc/users/next_whats_new/ if major new feature (follow instructions in README.rst there)
• Documented in doc/api/api_changes.rst if API changed in a backward-incompatible way