Remove AbstractOutPhaseSpacePoint

[AntonReinhard]

As discussed, the AbstractOutPhaseSpacePoint does not really serve any purpose and should therefore be removed.
However, I noticed that this makes the AbstractInPhaseSpacePoint equivalent to the normal AbstractPhaseSpacePoint because both would now require the incoming phase space to be non-empty. I would therefore propose to do the following:

• Allow an AbstractPhaseSpacePoint to have an empty out phase space, but don't allow an empty in phase space.
• Keep AbstractInPhaseSpacePoint as is, i.e., require the in phase space to be non-empty (which is now only an alias to AbstractPhaseSpacePoint since it also requires that). I think it would still be nice to have an explicit alias even though it's functionally the same.
• Remove AbstractOutPhaseSpacePoint.
• Add an AbstractFullPhaseSpacePoint (or something similar) that requires the incoming and outgoing phase space to be defined.