Deprecation of axes collision

The following is related to the PR #9037 (and incidentally to #9034 too).

Adding an axes instance to a figure by using the same arguments as for

a previous axes instance currently reuses the earlier instance. This

behavior has been deprecated in Matplotlib 2.1. In a future version

(likely 2.3), a *new* instance will always be created and returned.

Meanwhile, in such a situation, a deprecation warning is raised by

:class:`~matplotlib.figure.AxesStack`.

This warning can be suppressed, and the future behavior ensured, by passing

a *unique* label to each axes instance. See the docstring of

:meth:`~matplotlib.figure.Figure.add_axes` for more information.