Merged dd-cleanup from repository ianna
Make sure the Store is read-only. Make all the changes needed at a compact view construction.
Use unique pointers.
Initialize world postion in default constructor.
Merge pull request #4476 from ianna/dd-read-only-view

DetectorDescription/Core -- DD Read Only View
Merge remote branch 'origin/CMSSW_7_2_X' into CMSSW_7_0_X