Merge magnetic forward functions for dipoles

[santisoler]

Merge the two forward functions for magnetic fields of dipoles into a single one named dipole_magnetic that includes a new field argument. Make the dipole_magnetic function to ask magnetic moment vectors as a tuple of three arrays, where each one of these arrays. Update tests accordingly. Update the user guide page about forward modelling of prisms.

Relevant issues/PRs:

Fixes #450

Implements the same interface as #448