Additional layer between ParticleStateful and PhaseSpacePoint

[AntonReinhard]

As discussed offline, it would be useful to have an additional object layer between ParticleStateful and PhaseSpacePoint, something like a ParticleList. It would essentially be a wrapper around the current Tuples of in/out particles in the PhaseSpacePoint. As such it should introduce zero overhead to the existing definition, both space and time-wise. It would also allow the utility (such as the type checking/assembling, definitions, and construction from coordinates) to be used elsewhere, for example in QEDevents when generating multi-particle samples.

[AntonReinhard]

This issue should be transferred to QEDcore.jl once the refactoring is done there.