Merge the two forward functions for magnetic fields of rectangular
prisms into a single one named `prism_magnetic` that includes a new
`field` argument, mimicking the interface of the gravity forward
functions. Update tests accordingly.

Make the `prism_magnetic` function to ask magnetization vectors as
a tuple of three arrays, where each one of these arrays.

Ditch the usage of `prism_magnetic_component` in the forward modelling
pages, and change how the magnetization of the prisms is defined.

Replace the `cast` argument in the private functions for the `shape` of
the expected output arrays. Extend the docstrings of those private
functions.

Co-authored-by: India Uppal <[email protected]>