Add dipole magnetic fields in spherical coordinates

[leouieda]

Description of the desired feature:

After #51 is done and we have the gravity gradient components in spherical coordinates, the magnetic fields of dipoles in spherical coordinates would be very straight forward because the kernel functions are the same as the gravity gradients (see the code for the prism magnetic field). So this shouldn't be too much work to implement.

Are you willing to help implement and maintain this feature?

Maybe but it will be a long time before I can find the time to do this. So if anyone wants to have a go, please feel free to do so!