Enable all EM fields from the external source

Description

Added a flag to external_source_eval to decide the type of field, and call it with a different flag depending on the interaction.

Closes #837 (closed) #834 (closed)

Also, implemented the calculation of different EM fields from a given one, both for plane waves and bessel beams.

Still need to:

• Add output of dipolar B field to maxwell_dipole_field output.
• Enable average method of calcualating dipolar B field in addition to center-of-mass evaluation.

Checklist

• I have checked that my code follows the Octopus coding standards
• I have added tests for all the new features added in this request.