mappings: Removed no longer needed VegX-VegBIEN mappings

mappings/Makefile: Made VegCSV-VegBIEN.specimens.csv a non-derived map, since the VegX-VegCSV mapping is no longer used. This causes automatic creation of a for_review file.

plots inputs: Removed maps/.VegX.*.csv.last_cleanup

plots inputs: Remapped all VegX via maps to VegCSV. See steps at <https://projects.nceas.ucsb.edu/nceas/projects/bien/wiki/VegX-%3EVegCSV&gt;.

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.)