PR summary

This PR continues the work of #28658 and #28454, aiming to close #14168. (Feature request: Bivariate colormapping)

This is part one of the former PR, #29221. Please see #29221 for the previous discussion

Features included in this PR:

• A MultiNorm class. This is a subclass of colors.Normalize and holds n_variate norms.
• Testing of the MultiNorm class

Features not included in this PR:

• changes to colorizer.py needed to expose the MultiNorm class
• Exposes the functionality provided by MultiNorm together with BivarColormap and MultivarColormap to the plotting functions axes.imshow(...), axes.pcolor, and `axes.pcolormesh(...)
• Testing of the new plotting methods
• Examples in the docs