GeoModelKernel streamline classes

• GeoGraphNode: Do not change it w.r.t what's in main. We only need the parent for the fetching of the transforms
• GeoPlacement: Move the parent method here
• GeoVPhysVol: The handling of the child tree is implemented in GeoFullPhysVol & GeoPhysVol. I'd say that this is code duplication
• GeoPositionCacheNode: Template the class over the later usecase. The specified template will be later used to communicate with the GeoAlignmentStore. Remove the virutal function calls & Implement the changes from !314 (closed) w.r.t. transform retrieval.

Tagging: @boudreau, @tsulaia