schemas/VegCore/VegCore.ERD.mwb: added back aggregate_observation as a superclass of taxon_absence, taxon_presence. have only aggregate_observation, not taxon_observation, inherit from taxa_sampling_event because taxa_sampling_event is designed for multiple or aggregate samplings, but taxon_observation can be for just one occurrence. taxa_sampling_event: added required within_place since taxa must be sampled within a particular place. individual.place: renamed to within_place to clarify that each individual does not necessarily need its own place instance. individual_observation: added place_observed_at to serve the role that individual.place previously did (the place of an individual may change between observations).