Resolve "Provide VectorObservable for constituents of xAOD::CompositeParticle s"

Release notes

• fixed TQStringUtils::rfindFree returning nonsense in almost all cases when parenthesis were involved
• added the TQCPConstituentVectorObservable allowing to retrieve, e.g., the pT of all parts (or otherParts) of a CompositeParticle ("EventCandidates" in PAOD lingo), see http://atlas-caf.web.cern.ch/observables.html for details and usage.

Details

Closes #379 (closed)