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.
I guess this "if" chain should first select appropriate det() and only then test subdetId()