adding a spherical Earth model in Ellipsoid

[fbereux]

Hello,
I wonder if you would agree to add a spherical Earth model in the ellipsoid class (with semi-major and semi-minor axis equal to 6371 km). This would make it possible to define :
sphere = Ellipsoid(model="spherical_earth")
Such an addition would be useful to test some analytical geolocation algorithm based on spherical Earth model (for instance Ho & Chan) while using all the facilities provided by the pymap3d library (thank you to the developers !)

[fbereux]

by closer inspection to 'ellipsoid.py' I think 'wgs84_mean' is exactly what I was looking for ...