Add horizontal Cartesian components for point masses

[birocoles]

Implement horizontal Cartesian components g_northingand g_easting for point masses. This PR addresses some of #85 . Components in spherical coordinate system will be implemented in the future.

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.

[leouieda]

@birocoles notice that your branch is out of date with respect to the master branch. You might want to merge in the changes before proceeding (you can do this locally or through Github)

[birocoles]

Hi @santisoler! I may have done something wrong when I tried to pull things from master. Actually, I think the sanity checks are important. Whatever. I will bring this sanity check and the related test back to the code.

[birocoles]

@santisoler , I think everything is working now.

[birocoles]

I have just fixed some problems with too long lines. What I have to do now?

[santisoler]

Thanks for tackling this @birocoles! 🥇

I think this is good to be merged. If you agree, please remove the WIP from the PR title and ask @leouieda for a review so we can merge it.
Thanks again for the hard work!

[birocoles]

Thanks @santisoler

[leouieda]

@birocoles I'm happy with this PR 👍 Thanks for doing all this work! The numerical tests are really good.