Single axes artist

This adds check to ensure that you don't add an artist to more than one axes (which does not work because the transform stacks clobber each other).
I view this as an implementation detail which should only be called from the `add_*` methods on Axes (by c++ background is leaking through, Axes and Artist are friends) and probably never should have been part of the public API to begin with.

I was being as hedgey as possible in the removal date, I am in favor of doing it as soon as possible.