Normal gravity calculation

[leouieda]

Description of the desired feature

The normal gravity calculation should be done using the Li and Goetze (2001) formula that allows computation at any height. It would use the current ellipsoid (harmonica.get_ellipsoid) to do the calculations. This is what the function signature should look like:

def normal_gravity(latitude, height):
    ...
    return result

Latitude and heights can be any array like object so it's best to do the calculations with numpy functions instead of math.

It might be a bit tricky to test. One way is to compare the computed values at the equator and pole with the ones provided by the ReferenceEllipsoid (which are tested against Hofmann-Wellenhof & Moritz, 2006). That should probably be enough.