Implement clear() method for trigger menu, prescales, bunchgroups
In xAOD writing, existing trigger configuration structures are overwritten, rather than recreated. This lead to a duplication of information in the menu (which was reported by Kenji Hamano and traced by Tim Martin, see ATR-22614).
This merge request implements the proper clearing of all structures and adds a unit-test that checks if the single and double loading of the configurations work and gives identical results. The test loads a menu from the TriggerDB, if a TriggerDB connection can't be established (oracle access not available), it exits silently.
It resolves ATR-22614