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).

schemas/vegbien.sql: taxonconcept: Renamed morphospecies to taxonname since it's used in the same way as taxon.taxonname: to store the lowest-rank portion of the taxonomic name, such as the morphospecies suffix

inputs/.TNRS/tnrs/map.csv: Mapped *_matched terms that are both matched in the input name and which correspond to the matched taxonconcept (Genus_matched, Specific_epithet_matched, etc.) to both the input and matched taxonconcepts

inputs/.TNRS/tnrs/map.csv: Mapped terms matched in the original string (rather than deduced from the matched taxonconcept) to new verbatim* taxonomic terms

mappings/VegCore-VegBIEN.csv: Mapped verbatim* taxonomic terms to the TNRS input taxonconcept

mappings/VegCore-VegBIEN.csv: TNRS input taxonconcept: Split single _if statement controlling where morphospecies goes into two _if statements for each case, so that other verbatim* terms don't need to have an _if statement in their mapping to the input taxonconcept

mappings/VegCore.csv: Added back verbatim* taxonomic terms, which will now be used for the TNRS input taxonconcept. Note that they will have a different meaning than the original* taxonomic terms that they were renamed to in r5062.