inputs/.TNRS/tnrs/map.csv: Mapped Overall_score to new matchedTaxonFit_fraction

mappings/VegCore-VegBIEN.csv: Mapped matchedTaxonFit_fraction to _set_canon_taxonconcept(canon_concept_fit_fraction)

mappings/VegCore.csv: Added matchedTaxonFit_fraction

schemas/vegbien.sql: _set_canon_taxonconcept(): Also set the canon_concept_fit_fraction

schemas/vegbien.sql: taxonconcept: Added canon_concept_fit_fraction to store the closeness of fit of the canon_concept

schemas/vegbien.sql: taxonconcept: Renamed canon_taxonconcept_id to canon_concept_id to shorten the name, which is used often

sql.py: mk_update(): in_place: Convert columns of type character varying to text so that they can be merge-joined with text columns. Note that these two types are equivalent but not aliases of one another, so the explicit type change is needed.

sql_gen.py: Added canon_type()

sql.py: mk_update(): in_place: Factored retrieval of column type out into separate statement for clarity

schemas/functions.sql: _join*(): Fixed bug where was returning '' instead of NULL when only NULL inputs were provided, because array_to_string() always returns a non-NULL string. Functions must always return NULL in place of '' to ensure that empty strings do not find their way into VegBIEN, and to prevent inconsistencies between row-based and column-based import (row-based import folds empty strings to NULL while column-based import relies on having a clean input table).