Remap forward pixel DetIds in files from from old CMSSW versions

This is basically exactly the same as #4541, but with the commits cleared of everything other than salient ones (i.e. the ones to bring the branch up to date taken out).  #4541 seemed to be confusing github into thinking there were merge conflicts and more changes than there actually were, because CMSSW_6_2_X_SLHC had moved on in the meantime.

There are lots of plots on the original pull request with results from testing.