Update magnetic forward functions for dipoles

[santisoler]

Description of the desired feature:

After solving #446 and #447 with #448, we should apply similar changes to the dipole magnetic forward functions.

In a nutshell:

• merge the dipole_magnetic and dipole_magnetic_component functions into a single one,
• change the shape and type of the expected magnetic_moment argument: it should be a tuple of three arrays: mag_moment_e, mag_moment_n, mag_moment_u.

Are you willing to help implement and maintain this feature?

Sure!