mappings/VegCore-VegBIEN.csv: Remapped taxon hierarchy for accepted taxonconcepts to taxonconcept parent_id hierarchy

schemas/vegbien.sql: Fixed bug where taxonconcept.parent_id was missing a foreign key constraint

schemas/vegbien.sql: taxonconcept: Changed instructions for including a taxon name at a rank with no explicit column to create a parent taxonconcept for it and point to it using parent_id instead of using otherranks. Removed no longer used otherranks field.

schemas/vegbien.sql: taxonconcept: taxonconcept_required_key check constraint: Added taxonname

schemas/vegbien.sql: taxonconcept: taxonconcept_unique_within_creator_by_name unique index: Removed duplicate entry for creator_id

schemas/vegbien.sql: taxonconcept: Added parent_id to point to the parent taxonconcept

sql_gen.py: null_sentinels: Added 'unknown' for taxonrank

schemas/vegbien.sql: taxonrank: Added 'unknown'

mappings/VegCore-VegBIEN.csv: Also map *taxonRank to taxonconcept.rank, so that if it's in the taxonrank enum, it will automatically populate this field

mappings/VegCore-VegBIEN.csv: Remapped *infraspecificEpithet to new taxonconcept.taxonname rather than placing it in subspecies prefixed with the taxonRank, because it isn't necessarily the subspecies and because taxonname is defined to contain the lowest-rank portion of the taxonomic name. Note that when both morphospecies and infraspecificEpithet are provided, infraspecificEpithet takes priority for the taxonname field, because if TNRS leaves unmatched terms (which are tentatively mapped to morphospecies) but also matches an infraspecificEpithet, then the unmatched terms can't be for a morphospecies (because an infraspecificEpithet and therefore also a specificEpithet was matched, so the species is definite and formally named).