AthContainers, etc: Add getVector() methods to IConstAuxStore.

Add IConstAuxStore::getVector() return the IAuxTypeVector interface for the contained auxiliary variables.

I resisted adding this for quite a while, in order to avoid restricting how the AuxStores could be implemented. However, this turns out to be useful, and in fact every AuxStore implementation that's actually used maintains IAuxTypeVector interfaces to the auxiliary variables (and even for the one that currently doesn't, AuxStoreConstMem, we could implement enough to get things working, if needed).