Multithreading cleanup up in GeomDet

Add const to the declaration of a function
return type. The returned pointer was not
being used to modify anything. Eliminates
a static analyzer warning.
initialized var.multiplicity
Merge pull request #3832 from cms-btv-pog/minorFixMVAComputer

AT -- Initialized var.multiplicity
Merge pull request #3829 from wddgit/multithreadCleanupGeometryDet

Multithreading -- Cleanup up in GeomDet
Merge remote branch 'origin/CMSSW_7_1_X' into CMSSW_7_0_X