Description of the desired feature

The tesseroid modeling is done in geocentric spherical coordinates (longitude, geocentric latitude, radius) but the data is usually in geodetic coordinates (longitude, geodetic latitude, geometric height). Proj4 apparently only converts the latitudes (https://proj4.org/operations/conversions/geoc.html) but not the height to radius. We need a function that does this conversion so that we can use tesseroids appropriately.

The function will use the current ellipsoid (get_ellipsoid) to do the conversion. Tests can use simplified ellipsoids to check that the computations are correct (like setting the flattening to 1 and axis to an integer). It could go into a new harmonica/coordinates.py module and should look like:

def geodetic_to_geocentric(latitude, height):
    ...
    return geocentric_latitude, radius