Simple HGC Clustering and HGC GsfElectrons

@vandreev11 @pfs @amagnan @bsunanda

HGC Clustering and GsfElectrons as described in talks this week.
This has been made to work on the Valeri's latest change to store RecHit energies in GeV.
Hi Fedor, I agree that logic would be cleaner, but given that this code operates on super-clusters (where the data volume is significantly reduced) it makes no difference in performance.