Enhancement of phase space point

[szabo137]

The current implementation of ParticleStateful and PhaseSpacePoint could be improved. Here we collect some ideas:

particle stateful

• expose the types of the fields as type-parameters
• remove spin information from particle stateful (will be added to the process interface)

phase space point

• Think about saving only the momenta and wrapping the particle stateful type only on access,
• make generate_phase_space private and all it in a constructor of PhaseSpacePoint,
• add construction of phase space points from coordinates, maybe using the functions _generate_incoming_momenta and _generate_outgoing_momenta from the process interface.
• add abbreviations for momentum.(psp.in_particles) and momentum.(psp.out_particles

[AntonReinhard]

Fully solved with #63 and #68