subtract: Support "ragged rows" that have fewer columns than the specified column numbers

util.py: list_subset(): Added default param to specify the value to use for invalid indexes (if any)

mappings/VegX-VegBIEN.stems.csv: Mappings with multiple inputs for the same output: Use _alt, etc. to map the multiple inputs to different places in the XML tree, so that when using a pregenerated tree, the empty leaves for each input will not collide with each other

mappings/VegX-VegBIEN.stems.csv: Changed XPath references (using "$") to XML function references using _ref where needed to make them work even on a pre-made XML tree used by all rows

xml_func.py: Added _ref to retrieve a value from another XML node

xml_func.py: Made all functions take a 2nd node param, which contains the func node itself

bin/map: If outputting to a DB, also create output XML elements for NULL input values. This will help with the transition to using the same XML tree for all rows.

xml_func.py: _label: return None on empty input

mappings/VegX-VegBIEN.stems.csv: Added _collapse around subtrees that need to be removed if they are created around a NULL value

xml_func.py: Added _collapse to collapse a subtree if the "value" element in it is NULL