Add semimedium_axis property to sphere and ellipsoid for compatibility

[MarkWieczorek]

This PR adds the property semimedium_axisand semimajor_axis_longitude to the Sphere and Ellipsoid classes. This is done for compatibility. I note that the Sphere class already had semiminor and semimajor axes set.

Furthermore, I have removed "Added for compatibility with pymap3d" from the docstring, as I encountered this compatibility problem in pyshtools... In many cases, people will treat (Sphere, Ellipsoid, TriaxialEllipsoid) as just a boule instance, and won't care which "subclass" they are dealing with.

Relevant issues/PRs: None.