bugfix: mappings/VegCore-VegBIEN.csv: nest all taxonoccurrences inside a stratum event, so that the parent locationevent is always fully populated before child locationevents point to it. (previously, a stub parent event was created when the child event was imported first, which blocked the fully-populated parent event from being inserted later on.) this uses auto-folding (for VegBank/CVS) and auto-forwarding (for other datasources) to prune empty stratum events for taxonoccurrences that don't have strata. (see wiki.vegpath.org/Auto-folding, wiki.vegpath.org/Auto-forwarding for more info about these normalization techniques.) note that the inserted row counts stay exactly the same for all datasources except VegBank (which was being fixed), indicating that this signficant change to the mappings did not change the semantics of the import of taxonoccurrences.