Add a method for the ellipsoid geocentric radius

[leouieda]

This is the distance from the center of the ellipsoid to its surface as a function of geodetic latitude.

Fixes #34

Reminders:

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• [ ] Add tests for new features or tests that would have caught the bug that you're fixing.
• [ ] Add new public functions/methods/classes to doc/api/index.rst and the base __init__.py file for the package.
• [ ] Write detailed docstrings for all functions/classes/methods. It often helps to design better code if you write the docstrings first.
• [ ] If adding new functionality, add an example to the docstring, gallery, and/or tutorials.
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[leouieda]

@MarkWieczorek just to check, this is what you had in mind right?

[MarkWieczorek]

That is about what I was looking for: But would it be possible to allow inputting geodetic or geocentric latitude? Perhaps you could just add a default parameter like geodetic=True. If False, the geocentric latitude would first be converted to geodetic.

I have almost never used geodetic coordinates, and when dealing with spherical harmonics, it is natural to use the gecentric latitude :)

[leouieda]

Hum, I'll need to think about this a bit since we need the radius in the first place to convert geocentric to geodetic. So it doesn't seem to be straight forward. But there should be a way.

[leouieda]

Alright, I worked out the equation and implemented the option for geodetic=False and some tests. I added a note saying that this is an exception since we generally favor solutions where the conversions are done by the user prior to input to simplify the code. But this wouldn't be an option here since there is no way to do the conversion without knowing the radius already.

Seems like I got my math right :slightly_smiling_face: