xAODCoreAthenaPool: Fixes for writing ShallowAuxContainer with variable vetoes.
Vetoing variables from a ShallowAuxContainer had a couple problems.
- When testing for whether we can do a simple copy, we also need to check that there are no variable vetoes.
- Fix setting of number of elements to be copied in the case of variable vetoes but no thinning.
- When we copy variables, the new container should always be set for shallowIO. All the variables we want to write will be copied to the new container. Any that are in the parent but not the new container will have been explicitly vetoed.