[WIP] Added PHPCR purge

This PR provides `purgeRepository` as an analagous method to the ORM DBManagers new `purgeDatabase` method.

Note I havn't tested this yet. So it should not be merged.
Note this is inefficient. It will instantiate the Executor 2 times, one for purgeRepository and one later in this method.

I'm not sure what the overhead is... caching it is slightly awkward as it depends on if the executor should initalize the repository or not.