Add new AthenaPool converter able to write old persistent version

This converter is able to write two peristent representations depending on the value returned by a user-defined method writeOldPers(). Usually that would be the current version and one of the old versions. Given the Event data format used be default in Athena, the choice of which version to write should be fixed for a given job.

So far it is just a template that is not instantiated anywhere, so as long as it compiles it will not create any troubles.