join: Added map_1_core_only option that uses only columns 0 and 1 of map_1. This is useful for one-time refactoring joins where the Source column, mappings comments, etc. shouldn't be part of the datasource's via map (although they will be part of the autogenerated VegBIEN map)

join: Use opts.env_usage() for usage message

mappings: Made VegCSV-VegBIEN.{plots,organisms,stems}.csv symlinks to VegCSV-VegBIEN.specimens.csv

mappings/Makefile: VegCSV-VegBIEN.specimens.csv: Commented out combining with DwC2-VegBIEN mappings, because merging DwC and VegX/VegCSV into one map is a lower priority than replacing all datasource VegX mappings with VegCSV (which does not require the merging but does require XPaths that don't collide, which is not yet the case)

lib/xml_func.py: _if(): Made then param optional, so that user can just map to the else branch as a shortcut for logically inverting the condition. (Note that a _not() XML function does not exist yet, so this is also a workaround.)

VegBIEN mappings: Wrapped dates in _date() and _dateRangeStart()/_dateRangeEnd(), to assist in importing date and date range values that PostgreSQL cannot parse. This will increase the import time, but hopefully also decrease the # of invalid values in the errors tables. (These functions can later be optimized to reduce the impact on import time.)

sql_io.py: put_table(): is_literals: is_function: Fixed bug where function call needed to be recreated in each iteration of the main loop, because the arguments to the function, which are based on mapping, may change as the result of error handling replacing invalid values with NULL

sql_io.py: put_table(): is_literals: Fixed bug where sql.select() that calls the function needed to be run recoverably, to auto-rollback errors. Made sql.select() cacheable because SQL functions are immutable, so it should be idempotent.

mappings/DwC2-VegBIEN.specimens.csv: Remapped taxonRemarks to taxondetermination.notes because http://rs.tdwg.org/dwc/terms/#taxonRemarks indicates that these notes are "about the taxon", not the specimen/plant in general

mappings/DwC2-VegBIEN.specimens.csv: Remapped eventDate to new aggregateoccurrence.collectiondate, which is a more accurate place than locationevent.obsstartdate/obsenddate because the date refers to a specific specimen. This also makes eventDate compatible with plots data.