PR Summary

This PR is in referance to GSoC project '2D colormaps'.
(Project proposal is here)

• The BivariateColormap class subclasses Colormap class. After generation it is flattened to 1d look up table.

• The implementation of BivariateColormap is not perfect yet and just something has been generated to show proof of concept.

• The bivariate data is identified by ndim == 3 and if an instance of either Bivariate Colormap or Norm has been passed in. This data is normalized and mapped to univariate integer index so as to directly index flattened BivariateColormap.

• Plotting some air temperature and surface pressure data:
  

Feel free to review.

@story645 @tacaswell

PR Checklist

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