Made CaloCellGeometry interface const thread-safe

The previous implementation of CaloCellGeometry appears to have been
thread-safe, however the interface it used did not guarantee that.
This commit removes the mutables and makes any modifying member
methods non-const. No client code was disturbed by this change.
Merge pull request #3413 from Dr15Jones/makeCaloGeometryConstThreadSafe

Geometry -- Made CaloCellGeometry interface const thread-safe