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

schemas/vegbien.sql: definedvalue: Made definedvalue nullable so that each row of a datasource can have a uniform structure in VegBIEN, and to support reusing the same XML DOM tree for each row

xpath.py: Added is_xpath()

xml_dom.py: set_value(): If value is None and node is Element, remove value node entirely instead of setting node's value to None