schemas/VegCore/VegCore.ERD.mwb: taxon_observation: allow any taxon_observation to link directly to a taxa_sampling_event, not just aggregate_observation. this allows any taxon_observation's methodology information to be found directly by joining to taxa_sampling_event. previously, one had to join to event.parent and then taxa_sampling_event, and it was not clear that event.parent would even be a taxa_sampling_event.