AthContainers, etc: Move Accessor, etc. classes out of AuxElement.

Accessor and related classes have been members of AuxElement, the original thinking being that derived classes of AuxElement may want to redefine them. However, in retrospect, there does not seem to be any circumstance in which that would actually make sense. Further, it prevents having any specializations for these classes. So move these classes out of AuxElement into the SG namespace (but with the names brought into AuxElement via using declarations for backwards compatibility).