Add plot at BinningProcess level

[bmreiniger]

Closes #315

See the three new png test results, and the last cell of the modified doc/...FICO... notebook.

This draft adds the multi-plot to BinningProcess, and modifies the BinningTable.plot method to suit. We need to add the same sort of thing to the other single-var binners, ContinuousBinningTable.plot and MulticlassBinningTable, and reconcile any differences in how those three operate.

I'm not so familiar with the multidimensional, piecewise, or streaming binners; maybe something needs/should be done there too.

Other TODOs:

• write something to select the subplots grid shape; there's probably something we can borrow from pandas for this
• I'd really like to be able to share the y-axis of the response aggregates (WoE etc.); this would surface a quick feature importance.

(I wonder whether we can refactor a bit to make the various flavors share more code, and make this easier?...)