Enable E, B and A fields from external sources
The external sources at the moment can only provide electric field or vector potential to the partner systems. The B field is only available as an optional argument for the Maxwell plane wave boundaries, but it's not available for partner systems. In order to use E+B fields (multipolar expansion coupling) or A+B fields (full minimal coupling, including Zeeman) we need to be able to pass different types of fields through the various interactions.