Renaming and moving IdCont::IdentifiableContainer to ContainerAccessor
In order to avoid confusion with the current implementation of the IDC, the recently implemented class IdCont::IdentifiableContainer has been renamed as ContainerAccessor, since this is used as a wrapper to access elements in a xAOD container, and moved under a ActsUtils package.
The location might be changed in the future if needed.
Tagging some people: @pagessin @asalzbur @akraszna @ssnider @jmasik @tbold