Adding dipolar magnetic field for Earth computations

WARNING: The dipolar field can be imposed in two ways:
1. By adding a J_ext (rotational of B_ext) contribution to the Jhat term on the source.
2. By impossing the B_ext dipole field on the B field in the image side of the solver and on the particle mover.

The present commit uses the 1st method to impose the dipolar field. The second method does not work in its current state. Corrections will be performed in the following commits.