sql_io.py: null_strs: Added 'NULL', which is used by phpMyAdmin as the default "Replace NULL with" value for CSV exports

sql_io.py: cleanup_table(): Refactored to use for loop with array constant, so that additional NULL-equivalent strings can easily be added

mappings/roots/: Merged roots for different tables into one mappings/root.sh for Veg+, which handles all tables' mappings to VegBIEN

sql_io.py: put_table(): When ignoring all rows for an iteration, return literal NULL value instead of column of NULLs as an optimization for callers using that iteration's pkeys

inputs/import.stats.xls: Updated with stats from latest import

mappings/VegCore-VegBIEN.csv: Primary taxondetermination: Removed [role=identifier] because the role of the entity making the determination is unknown. Added [!isoriginal] filter to those mappings to ensure that primary taxondetermination XPaths map to a different taxondetermination than the [isoriginal=true] determination when both are present.

inputs/SALVIAS*/1.organisms/map.csv: Remapped cfaff to identificationQualifier, because it was previously mapped to the same taxondetermination as the Orig* terms but does not have a corresponding Orig prefix to indicate that it should apply to the original determination instead of the primary TNRS one

mappings/Veg+.terms.csv: Removed no longer used computer.* taxonomic terms

mappings/VegCore-VegBIEN.csv: Removed no longer used computer.* taxonomic terms

inputs: Regenerated VegBIEN.csv for several datasources, which had apparently not gotten regenerated when make was run after the taxonRank mapping addition