README.TXT: Schema changes: Updated filenames of PDF ERD exports

Regenerated vegbien.ERD exports

xpath.py: parse(): _value(): Support '+' as a word character that doesn't need to be quoted

intersect: Fixed bug where test for ignore option needed to be removed, because ignore is not supported by this program

util.py: list_subset(): Fixed bug where using '+' to append the rest of the list didn't work if '+' was the first index, because max() cannot be called on an empty list

mappings/DwC2-VegBIEN.specimens.csv: Added VegCSV mappings, to enable use of one VegCSV-VegBIEN mapping for specimens and plots data

inputs/XAL/maps/DwC.specimens.csv: Remapped FieldNumber to recordNumber because this historical DwC term (http://rs.tdwg.org/dwc/terms/history/index.htm#fieldNumber-2009-04-24) has close to the same meaning as recordNumber, but not the same meaning as the current fieldNumber term

inputs/SpeciesLink/maps/DwC.specimens.csv: Remapped fieldNumber to recordNumber because term usage was inconsistent with DwC definition. Datasources often confuse this term, because it seems like the collection number, but is actually the author code for the event (VegBank's authorObsCode).

mappings/DwC2-VegBIEN.specimens.csv: catalogNumber: Added additional VegCSV mappings for mergability. taxonoccurrence.authortaxoncode: Added alternative mappings from VegCSV for mergability.

xml_func.py: simplify(): Apply pass-through optimizations for _if statements with no condition (which means false). This faciliates automated testing after an _if statement has been added, because the put template provided as part of the automated test will only change for those datasources that actually have a condition entry for the _if statement, which greatly reduces the number of tests that need to be accepted. (Note that the path before the _if will still be included as an empty path if there are no other mappings to that table, because the _if statement does not surround it.)