Process distribution interface

[szabo137]

This PR adds an interface for distributions associated with the triple scattering processes, compute models, and phase space definitions. The samples of these distributions are QEDcore.PhaseSpacePoints.

Summary of the changes/adds:

• New root type for ParticleSampleable with ProcessLikeVariate (already part of QEDevents): ScatteringProcessSampler
• states the required accessor functions for processes, models, and phase space definition
• adds respective _assert_valid_input_type
• adds respective Base.eltype
• adds respective Distributions.rand based on _randmom
• adds respective unit tests

Additionally, some rearrangements are done as well.

[AntonReinhard]

Looks good to me! Can't wait to us this together with QEDprocesses 👍