Fix issue with matplotlib 3.6.0 - #191

I'm converting this to draft for now b/c I want to add a simple regression test that compares images before we move forward.

Without the fix here the tests capture 18 failures, all related to the "pacman" reported #190:

@scls19fr this one is ready for review!

This fix may cause issues with older versions of Matplotilb, see https://matplotlib.org/stable/api/prev_api_changes/api_changes_3.6.0.html#axes-subclasses-should-override-clear-instead-of-cla

Looking at what cartopy does may be a good reference.

This fix may cause issues with older versions of Matplotilb, see https://matplotlib.org/stable/api/prev_api_changes/api_changes_3.6.0.html#axes-subclasses-should-override-clear-instead-of-cla

Looking at what cartopy does may be a good reference.

I tested with 3.5 but I'll test a bit more and pin mpl to a >= ver_that_work. Thanks!