Add Ellipsoid.normal_gravitation() method

[MarkWieczorek]

The Sphere class has the option of computing either the normal gravity or normal gravitation. In contrast, the Ellipsoid class only gives the option to compute the normal gravity. I propose that we allow the computation of the normal gravitation of an ellipsoid by creating the method Ellipsoid.normal_gravitation. This would help make the two classes consistent. Furthermore, there are cases (such as when you are not attached to the Earth) when the normal gravitation would be more appropriate.

[leouieda]

I completely agree! The main issue is that this will require some deductions first. Standard books like Physical Geodesy by Hofmann-Wellenhof & Moritz (2006) don't have the equations. There is an equation for the gravitational potential but they take the derivatives of it after adding the rotational term. So we'd first need to do the maths of deriving the equations. See #151 for references.