Add conversion system for phase-space element types

[szabo137]

In #39 the evaluation of diffCS on coordinates was added, which is essentially just a wrapper around the evaluation on four-momenta. This includes a conversion of a vector of reals (referred to as coordinates) to a vector of four-momenta by using the function _generate_momenta. As @AntonReinhard suggests, we should extend this idea to a full conversion system, which takes phase-space points with any element type and converts them into four-momenta. For four-momenta itself, the conversion is then just the identity.

The wrapper could then look like

function diffCS(ps::AbstactVector{T}) where {T}
    input_momenta = convert_phase_space_point(ps,SFourMomentum)
    return _diffCS_kernel(input_momenta)
end

where _diffCS_kernel is the internal version of diffCS evaluated only on four-momenta and using the differential cross-section interface (see #38 for detail).

An open question is the treatment of phase space definition in this context; This issue is therefore somewhat related to QEDjl-project/QEDbase.jl#139.

Originally posted by @AntonReinhard in #39 (review)

[AntonReinhard]

With the full implementation of the PhaseSpacePoint type, this is solved now. See #51, #63, and #68. It serves as a common internal representation of phase space points, and any representations can be converted to it in a specific constructor.