Update gem digitizer for ge21 hits

This commit is to ensure that the GEM digitizer can handle GE2/1 hits as well. It is built on top of https://github.com/cms-sw/cmssw/pull/2056. The merge conflicts are due to the merge conflicts in #2056. The recipe to resolve the conflicts is provided there. Currently the noise rates values for GE2/1 are taken from GE1/1. They will be updated once we have results from GE2/1 simulations.