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.

… 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.