AthContainersRoot: Update clear to work on AuxVectorData.

Update the clear method of IAuxTypeVectorFactory / AuxTypeRegistry to take
AuxVectorData instances rather than bare pointers.  Remove the clear static
method from AuxTypeVector.
Also alter clear to operate on a range, rather than a single element.

Refactoring to make possible dependencies between auxiliary variables,
as would be used to support packed links / jagged vectors.