Rework _generate_momenta interface

[AntonReinhard]

As it stands currently, the _generate_momenta interface cannot be implemented physically.

The functions will need to be:

    _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{N,T},
    ) where {N,T<:Real}

I.e., the outgoing momenta need both in and out phase spaces. The derived function then needs to be changed correspondingly:

function _generate_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}
    in_momenta = _generate_incoming_momenta(proc, model, phase_space_def, in_phase_space)
    out_momenta = _generate_outgoing_momenta(proc, model, phase_space_def, in_phase_space, out_phase_space)

    return in_momenta, out_momenta
end

(Opened from discussion on #68)