Add linearity tests for forward models

[manduinca]

Fixes #293

Adds test/test_linearity.py with linearity tests for each geometry (points, prisms, tesseroids), parametrized by field, following the discussion in the issue: the field of a set of sources is compared against the sum of the fields of each source computed separately.

Includes both gravity and magnetic forward models: point_gravity (Cartesian and spherical coordinates), dipole_magnetic, prism_gravity, prism_magnetic and tesseroid_gravity, using mixed-sign masses/densities/magnetizations.

The newer ellipsoid_gravity/ellipsoid_magnetic forwards could get the same treatment in a follow-up if you'd like.

[santisoler]

Hi @manduinca! Sorry for the delayed review. I was busy at a conference and catching up with stuff.

This looks really nice, thanks again for taking the effort and work on it! I left some minor suggestions below. Nothing major, just to make a few portions of the tests more Pythonic. Please, let me know what do you think, and feel free to apply and modify the suggestions as you like.

[santisoler]

Minor suggestion here: we don't need to index the arrays, we can iterate over them with a zip function:

Suggested change

	sample_prisms[i : i + 1],
	sample_densities[i : i + 1],
	field,
	)
	for i in range(sample_densities.size)
	prism,
	density,
	field,
	)
	for prism, density in zip(sample_prisms, sample_densities, strict=True)

[santisoler]

Similarly here:

Suggested change

	sample_tesseroids[i : i + 1],
	sample_densities[i : i + 1],
	field=field,
	)
	for i in range(sample_densities.size)
	tesseroid,
	density,
	field=field,
	)
	for tesseroid, density in zip(sample_tesseroids, sample_densities, strict=True)

[santisoler]

I would suggest indexing the arrays directly, so we pass only a float for each coordinate and for the mass. This way we are also testing (although we are probably already testing that in other places) that passing just a float as coordinates and single float for mass also works.

Suggested change

	tuple(p[i : i + 1] for p in sample_points),
	sample_masses[i : i + 1],
	tuple(p[i] for p in sample_points),
	sample_masses[i],

[santisoler]

Similarly here:

Suggested change

	tuple(p[i : i + 1] for p in points),
	sample_masses[i : i + 1],
	tuple(p[i] for p in points),
	sample_masses[i],

[santisoler]

And here:

Suggested change

	tuple(d[i : i + 1] for d in sample_dipoles),
	tuple(m[i : i + 1] for m in sample_magnetic_moments),
	tuple(d[i] for d in sample_dipoles),
	tuple(m[i] for m in sample_magnetic_moments),

[santisoler]

Suggested change

	sample_prisms[i : i + 1],
	tuple(m[i : i + 1] for m in sample_magnetizations),
	sample_prisms[i],
	tuple(m[i] for m in sample_magnetizations),