Generalize process, model, phase_space_definition

[szabo137]

Currently, there are three functions process, model, and phase_space_definition, which are interface functions for phase space points. However, the concept is more general, because there are other entities associated with scattering processes, computational models, and phase space definitions. For instance, certain samplers of QEDevents are also associated with these properties. Therefore, I propose to move these functions to a more general place in the code, maybe near the associated return type. Furthermore, I suggest to update the docstrings to reflect the more general usage of these functions.

Note: this change will not be breaking, because these are just exported symbols anyway.