Documentation for generate momenta interface

[AntonReinhard]

The documentation for the AbstractProcessDefinition should mention that the functions

_generate_incoming_momenta(
    proc::AbstractProcessDefinition,
    model::AbstractModelDefinition,
    phase_space_def::AbstractPhasespaceDefinition,
    in_phase_space::NTuple{N,T},
) where {N,T<:Real}

_generate_outgoing_momenta(
    proc::AbstractProcessDefinition,
    model::AbstractModelDefinition,
    phase_space_def::AbstractPhasespaceDefinition,
    in_phase_space::NTuple{N,T},
    out_phase_space::NTuple{M,T},
) where {N,M,T<:Real}

need to be implemented to generate PhaseSpacePoints from coordinates of a specific PhasespaceDefinition.