AthContainers, etc: 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.