mappings/Makefile: human-readable maps in for_review: Simplify just the output column so that the input column can be programmatically linked back to the original input names/XPaths

mappings/Makefile: Removed no longer used $(chRoot), $(cpReview)

Removed the human-readable mappings mappings/for_review/VegX-VegBIEN.plots.csv, VegX-VegBIEN.organisms.csv because these are now duplicates of VegX-VegBIEN.stems.csv

review: Support limiting the XPath simplifying to custom columns, rather than always the first two

review: Usage message: Fixed typo

Added mappings/for_review/VegBIEN-DwC2.specimens.csv, generated by inverting for_review/DwC2-VegBIEN.specimens.csv. This will be used to help translate VegX->VegCSV.

mappings: Made VegX-VegBIEN.organisms.csv, VegX-VegBIEN.plots.csv symlinks to VegX-VegBIEN.stems.csv instead of building them in the Makefile by copying VegX-VegBIEN.stems.csv, since these files are now always the same

mappings/VegX-VegBIEN.stems.csv: _if that maps to specimenreplicate via plantobservation or voucher: Refactored to map right-hand side of _eq in the left-hand side mapping, rather than in all then/else mappings. Distinguish this _if statement from others using new name param.

xml_func.py: _if(): Documented that can add `name` param to distinguish separate _if statements

xml_func.py: _if(): Made cond optional. When it's not specified or None, it is treated as False. This supports cases where all elements of the condition are required but not mapped to.