Add functors (nPVs, nTracks, nFTClusters) for reading RecSummary
Add functors for reading LHCb::RecSummary object. The set of MRs that followed before regarding persistency of this object from HLT2 stage onwards, currently only fill nPVs, nTracks and nFTClusters. So add functors for accessing these only. Closes #366 (closed)
To be tested with: DaVinci!729 (merged)