GeoModelHelpers -- Introduce cloneVolume

Introduce new function to clone physical volumes.

• If a fullphysical volume in the subtree is detected, then a clone of the volume is made
• Otherwise, it's steered via the boolean argument

The question is what are we doing with the clone methods of the GeoPhysVol & the GeoFullPhysVol. They don't have this detection mechanism. I'd propose to remove them

Tagging: @mbandier, @boudreau, @tsulaia, @rbianchi