xml_func.py: process(): Changed rel_funcs param to a callback is_rel_func, so that caller can specify any dynamic function to determine if a name is a relational function rather than having to list out all known relational functions

sql.py: function_exists(): Use simpler cast to regproc instead of query of information_schema.routines to determine if function exists. When the schema is not specified, this also limits the schemas checked to the search_path instead of the whole DB.

schemas/functions.sql, py_functions.sql: Renamed trigger functions to avoid collisions with plain SQL functions of the same name but different signatures, so that the plain SQL functions can be uniquely identified by their name without also requiring their signature

sql.py: mk_select(): In queries without a FROM clause, don't order by pkey

sql.py: mk_select(): Support queries without a FROM clause

sql.py: Added DoesNotExistException and parse it in run_query()

sql_io.py: put_table(): Removed no longer used conds var (invalid rows are removed from the in_table using sql.delete() instead of being filtered out in the main select)

sql_io.py: put_table(): Removed no longer used distinct_on var (sql.distinct_table() handles filtering the join_cols)

schemas/py_functions.sql: _date(): Just run str() on the returned datetime because it will usually be converted to a PostgreSQL timestamp anyway, so excluding the time from the string isn't necessary

schemas/py_functions.sql: Added _date()