AthContainers+AthContainersInterfaces: Add metafunctions to help choose the correct pool converters.

Add a supportsThinning.h containing a couple metafunctions to help determine which pool converter should be used for a given type. Add supportsThinning flag to xAOD base classes.

In copyAuxStoreThinned, don't try to resize to 0 if we don't need to. Prevents issues when applied to standalone objects.

Add the flag IAuxStore::supportsThinning.