Reinstate addAlignable in TransformProcessor for GeoModelXML

In a recent update (!128 (merged)) a call to addAlignable was removed:

Fix crash on hardcoded position of PV by commenting call to useless addAlignable (only prints log msgs)

While it is true that in standalone GeoModel this only prints log messages, when run within Athena this is not the case as this virtual function will call the version from the derived classes defined in Athena, which does add the AlignableTransform as it intended.

Without this call, we will not be able to add any AlignableTransforms to our DetectorManagers.

It seems that updates for the removal of mandatory name tags broke the access to the FullPhysVol via the previous method, and so a different solution will be needed to access these.

FYI @rbianchi