Add linearity tests for forward models

[santisoler]

Description of the desired feature:

Our forward models for the different geometries (points, prisms, tesseroids) allow to take multiple bodies as inputs, returning a single array as the resulting field generated by every one of these bodies on the computation points.
One of the properties of these forward models is their linearity: the resulting field for all the bodies should be equal to the sum of the individual fields.
Although this property is easily readable from the code, it might be wise to add functions that actually test them.
We could add test_linearity functions for each geometry that compares the resulting field of passing a set of bodies to the resulting field of computing the fields of each one separately and then sum them up.

Through pytests.mark.parametrize we could write a single function per geometry, adding the field as a parameter.
For example:

@pytest.mark.parametrize("field", ("potential", "g_z", ...))
def test_linearity(field):
    """
    Test linearity of the forward model
    """
    ...

Are you willing to help implement and maintain this feature? Yes, but I would love to help anyone motivated to tackle it. 💪🏼