schemas/VegCore/VegCore.ERD.mwb: added taxon_occurrence to store the underlying occurrence for each taxon_observation. we cannot use reobservable for this because not all taxon_observations are reobservable, but the one-time observations still have an associated occurrence. moved reobservable.current_determination to taxon_occurrence and renamed it to current_observation (this can be used to find the latest observation for each occurrence).