Particle Flow for HGC --  for SLHC16

This PR contains a preliminary version of particle flow that includes the HGC.
Charged/neutral hadrons and photons are now reconstructed in the HGCal, but many details need to be filled in. Track-cluster linking is in place in this version so charged pions can be studied and refined.

This PR includes #4551 from @pfs, but should merge properly if #4551 goes in first.

Barring any problems that the present testing (min bias, simple jet gun, pion gun, electrons, pi0s) hasn't found, this should go in SLHC16.

@vandreev11 @pfs  @amagnan