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

xml_dom.py: Added value_node(). Use new value_node() in value() and set_value(). set_value(): If the node already has a value node, reuse it instead of appending a new value node.

xpath.py: put_obj(): Return the id_attr_node using get_1() because it should only be one node

xml_func.py: _simplifyPath: Also treat the elem as empty if the required node exists but is empty

db_xml.py: put_table(): Added part of put() code that should be common to both functions

xpath.py: put_obj(): Return a tuple of the inserted node and the id attr node

xpath.py: set_id(): When creating the id_path, use obj() (which deepcopy()s the entire path) because it prevents pointers w/o targets

xpath.py: set_id(): When creating the id_path, deepcopy() the id_elem because its keys will change in the main copy