Tesseroids

[mdtanker]

WIP for updating some code relevant to tesseroids.

Relevant issues/PRs:

[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.