ENH: add colorbar info to gridspec cbar

[jklymak]

PR Summary

Non-gridspec colorbar axes have a _colorbar_info appended to them, but the gridspec and user-supplied ones do not. This PR adds the attribute to all colorbar axes, and is set to 'user' if the axes is provided by the user.

This is helpful in cases where we want to tell if an axes is a colorbar or not, though all colorbar axes should also be of type ColorbarAxes, so maybe not strictly needed...

PR Checklist

• Has pytest style unit tests (and pytest passes).
• Is Flake 8 compliant (run flake8 on changed files to check).
• New features are documented, with examples if plot related.
• Documentation is sphinx and numpydoc compliant (the docs should build without error).
• Conforms to Matplotlib style conventions (install flake8-docstrings and run flake8 --docstring-convention=all).
• New features have an entry in doc/users/next_whats_new/ (follow instructions in README.rst there).
• API changes documented in doc/api/next_api_changes/ (follow instructions in README.rst there).

[QuLogic]

This doesn't appear to be read anywhere? Is that a bug elsewhere?

[jklymak]

No the point is just to be consistent with the non-gridspec axes. In particular we can now check for the attribute. However, after I put this in I realized that we can check if an axes is a colorbar more simply just by whether it is a ColorbarAxes, so maybe this isn't really needed.

[QuLogic]

Well, I only meant panchor, but if you think there's a better way to do this entirely..

[jklymak]

Oh I see. I included panchor just as a precaution - we may want to handle it better in the future.

[timhoffm]

While dynamically adding attributes as needed is possible in python, people have found that it's hard to use because you never know which attributes are available (and when) on an object. And getattr/ hasattr are much harder to use than self._colrobar_info is None.

Therefore +1 for making this a regular field of a ColorbarAxes.