Add centrifugal potential to Ellipsoid and Sphere

[MarkWieczorek]

This PR allows one to compute the centrifugal potential of a rotating Sphere of Ellipsoid. The equations are simple. For the Ellipsoid class, I used the definition of the prime radius of curvature to compute the perpendicular distance to the rotation axis: $(N(\phi) + h) \cos\phi$, where $\phi$ is geodetic latitude and $h$ is ellipsoidal height.

A separate PR will implement this for triaxial ellipsoids, as we will need to change how we handle longitude_semimajor_axis.

Relevant issues/PRs:
Relevant to #151

[MarkWieczorek]

Looking for advice on how to "test" this routine. One possibility would be to check that the centrifugal potential of an ellipsoid with zero flattening is equal to that obtain from the Sphere class.

[MarkWieczorek]

This PR was incorporated into #187