Provide VectorObservable for constituents of xAOD::CompositeParticle s

We currently have no elegant way (i.e. Factory based) to get a VectorObservable with pT /eta/phi/type/... for all entries in the "parts" or "otherParts" of a CompositeParticle.

TODO: add such a method. It should be noted that the type of the constituent is important such that one can use the FilterObservable to then select only, e.g., jets.