Documentation for base_state and propagator

[AntonReinhard]

The doc strings for base_state and propagator should be updated to reflect that they're interface functions supposed to be implemented by QEDcore (or another implementing package)

Also, the signature of propagator allows an optional mass value to be passed, which is not actually implemented in QEDcore, probably because the mass is known from the AbstractParticleType's mass function anyways, so the parameter is unnecessary.