Forward modelling for point masses in Cartesian coordiantes

[santisoler]

Extend point_mass_gravity() to compute gravitational fields of point masses in Cartesian
coordinates. Add needed Numba jitted kernel functions in Cartesian coordinates. Add
coordinate_system argument to point_mass_gravity(). Add tests for the extended feature.

Fixes #48

Reminders

• Run make format and make check to make sure the code follows the style guide.
• 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.
• Write detailed docstrings for all functions/methods.
• If adding new functionality, add an example to the docstring, gallery, and/or tutorials.

[santisoler]

@leouieda Could you take a quick look at this. Specially to the design of the extended point_mass_gravity() function. I'm not quite sure about the dictionaries I defined, so I wanted to have a second opinion.

I added some functions to compute distances in Cartesian coordinates inside forward/point_mass.py. I intend to move them as proposed in #69.

If you can't find time to review it, let me know. I think the main issues that can live here are related with design, so maybe I could merge it and we can discuss it afterwards when you have time.

[leouieda]

Looks good! It would be good to have a gallery example as well to visually verify that the results are correct. Maybe one with 2 masses at different depths with opposite densities?

[santisoler]

Thanks for the quick review @leouieda. I've added a simple example as you proposed. If you like it, feel free to merge this.

[leouieda]

@santisoler really good! I like this new API. I made a few suggestions for the recipe.

[leouieda]

It would be good to elaborate a bit more on the docstring (citation, definitions, equations [?]).

[santisoler]

Yes, absolutely! I tend to forget to do it because I know where I'm getting the equations from, but I really appreciate finding math on the docstrings when I try to use a new library or function.

[santisoler]

I added some math and references. I think we must make clearer that the third coordinate in Cartesian system is pointing downwards.

[santisoler]

The warning mentioned above should be enough for making it very clear.

[santisoler]

@leouieda Would you mind taking a final look at this? If you agree with the new variable name and don't have any other suggestion, I think this is ready to be merged.

[leouieda]

@santisoler this is great! I love that docstring! Made a few corrections to the text but once these are done feel free to merge!

[santisoler]

Thanks for the review @leouieda! I like how it looks, so I'll merge it right away!