Add Magnetic current sources

[smartalecH]

Currently, the amplitudes_for_source() function allows the user to specify an arbitrary electric current-source distribution (e.g. $J_x$, $J_y$, $J_z$). It would be useful to also be able to specify accompanying magnetic current sources (e.g. $M_x$, $M_y$, $M_z$). This would enable injecting unidirectional profiles into a simulation (which requires a complete ansatz).