Replace GeoPVLink & GeoPVConstLink with a GeoSmartLink pointer

Hi everybody,

in atlas/athena!67229 (merged), @tsulaia told me that the GeoPVLink and the GeoPVConstLink are not incrementing / decrementing the reference counters of the PhysVols that they hold. In fact, they're nothing else than a wrapper. That's quite surprising to me and I think that this beheviour should change. In this MR, I've replaced the two classes by the GeoSmartLink class -- Improvements to the name are more than welcome -- which takes over the reference counting and also which can transparently casted into the bare pointers. The reference count updates will probably introduce some overhead to the overall code. How much I don't know.. How to test? I don't know neither.

Tagging: @rbianchi, @boudreau, @nstyles, @mbandier