sql.py: not_null_col: Changed value to 'not_null_col' so that column doesn't seem like a status indicator of whether some value is not null (in fact it's just a column that is always not null)

xml_func.py: Replaced xpath.get_1() with xpath.get_value() where possible, for simplicity

xml_func.py: strip(): Evaluate structural functions like _ignore and _ref by process() instead of removing them. Store structural functions' names in structural_funcs module var. This ensures that _ref targets are still expanded in column-based import.

xpath.py: get(): Create attrs: Put keys last so that any lookahead assertion's path will be created last as it would have without the assertion. This ensures that any value argument of an XML function will always go last even if a lookahead assertion would otherwise have caused it to be created with the element's keys, which previously were created before the attributes.

sql.py: put_table(): If is_func, default into table name ends in () instead of '-pkeys'

schemas/vegbien.sql, functions.sql: Made cast functions STRICT to enable the RETURNS NULL ON NULL INPUT optimization

db_xml.py: put_table(): Pass is_func to sql.put_table()

sql.py: put_table(): Added is_func param for whether out_table is the name of a SQL function, not a table

db_xml.py: put_table(): Treat every node name that starts with "_" as a function, not just members of put_table_special_funcs. This ensures that DB function args are always treated as values, not children with fkeys to parent.

bin/map: by_col: Strip only XML functions that are not in the DB