Removed mutable from HcalGeometry

One of the mutable member data of HcalGeometry did not need to be
mutable. This avoids thread-safety issues.
Merge pull request #2856 from Dr15Jones/removeMutableFromHcalGeometry

Multithreading fixes -- Removed mutable from HcalGeometry
Merge remote branch 'origin/CMSSW_7_1_X' into CMSSW_7_0_X