Add the HGCal calorimeter collections to the pileup mix

The mixing module only adds collections from the minimum bias files that it is told to into the pileup events.  If it is not told to add a collection, then it will not be available to the accumulators in the PileUpEventPrincipal, even if the collection is in the input file.  All collections for the signal event are always available.

Before this pull request only calorimeter hits from the signal event were digitised by the HGCal digitisers.

@lgray, @pfs