Update and refactor code and docs for tesseroids

[mdtanker]

This PR fixes and clarifies the documentation about how to use tesseroids and tesseroid layers. It also tries to align the functionality of prisms and tesseroids, by adding a _discard_thin_tesseroids() function.

[santisoler]

Thanks @mdtanker for revisiting the tesseroid code! Since it involves several files, maybe we could reorganize them similarly to how I organized the ellipsoids code. Since the harmonica._forward module is private, we can reorganize files as we please. We just need to remember to import the public functions in harmonica/__init__.py.

[mdtanker]

Okay that sounds good, I'll try and follow the ellipsoids code. For now, I was just ensuring the tesseroids code matches the prisms code as best as possible.

[santisoler]

Hi @mdtanker! I think there are many good things in this PR. I can leave the reorganization of the tesseroid source files for a different one, don't worry about that. Is there anything you want to work on in this PR?

I also noticed that you pushed a test.ipynb to this branch. Is that intended or should we remove it?

[mdtanker]

I think there were a few other things I may have not push yet but probably won't be able to get around to it for a week or 2.

[santisoler]

Sounds good, and no rush on my end. Let me know if you need another set of eyes here.

[mdtanker]

I git reset --hard to before you merged in main, then git reset --soft HEAD~1 to remove my WIP commit so I could separate it into individual commits, and then git push --force-with-lease the changes here. How should I merge main back in now, just git checkout tesseroids, git merge main?

I did that, and it seemed to work fine!

[mdtanker]

Ok @santisoler I think this is ready for review, but no rush. I had to update the outputs of one of the doctests (fb47337) but I'm not sure why any of my code changes would have affected this doctest, maybe it was already failing before?

[santisoler]

Looks good to me, @mdtanker! I just left a few minor comments below. Feel free to commit them and merge the PR!

When merging, we use Squash and Merge, and keep the PR comment as commit description. In this case, you might want to edit it to describe what has been done here. We usually remove the Relevant issues/PRs section, since it's not very informative outside of GitHub.