Include a reference Sphere class

[leouieda]

As shown in #13, flattening of 0 leads to not-a-number in the computations due to division by the first eccentricity (which is 0). In these cases, it would make more sense to have a boule.Sphere class that mirrors the Ellipsoid but is much simpler:

Sphere(radius=..., geocentric_grav_const=..., angular_velocity=...)

All the eccentricities, gravity at the pole and equator, and coordinate conversions aren't needed. The forward modelling equations are also simpler (should be implemented in Sphere.normal_gravity).

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[MarkWieczorek]

I think that a Sphere class would be very useful. In particular, I would like to add the reference spheres for Mercury, Venus, and the Moon. For these objects, the rotation rate is very small, and the non-hydrostatic component of the degree-2 shape is larger than the theoretical hydrostatic shape.