Functions should not use PhaseSpacePoint directly

[AntonReinhard]

This is not urgent, but the implementations of the interface functions such as _matrix_element currently use the concrete type PhaseSpacePoint from QEDcore, but should rather use the AbstractPhaseSpacePoint from QEDbase and its interface functions.

Related to this, they should not use the function _momentum_type since it is not part of the interface. Alternatively, we could make it part of the interface. However, since the momentum element type is not part of the AbstractPhaseSpacePoint's type parameters, I'm not sure this would make sense.