Scott Snyder authored Apr 02, 2024 and Vakhtang Tsulaia committed Apr 02, 2024

AthContainers: 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).