implemented inverse transform for Mollweide axes
added unit tests for Mollweide transforms

The new test functions test_mollweide_forward_inverse_closure and test_mollweide_inverse_forward_closure test that the forward->inverse and inverse->forward compositions are approximate identities.
use ax.transProjection instead of ax.transData in Mollweide axes unit test

The goal was to exercise the MollweideAxes.MollweideTransform and MollweideAxes.InvertedMollweideTransform classes. The ax.transData property does not make use of either of these classes; the ax.transProjection property does.
reduce precision to the level at which the test passes
Merge pull request #1624 from lpsinger/mollweide_inverse_transform

implemented inverse transform for Mollweide axes