sql_io.py: put_table(): Optimization for only literal values: Also support an empty in_tables list, for use by put()

sql_io.py: put_table(): Added optimization for only literal values, which does the same operations as put() but with the additional error handling of put_table()

pg_dump_vegbien: Don't use SET SESSION AUTHORIZATION because it doesn't work with the py_functions schema (it requires PL/Python functions to be created as user postgres and then the owner changed to bien, which SET SESSION AUTHORIZATION won't do)

sql_gen.py: Added is_literal() and use it where isinstance(..., Literal) is used

db_xml.py: put_table(): Divide fields into input columns and literal values: Translate values: Allow literal values other than strings or None (from the XML parsing), because sql_io.put_table() is getting an optimization for iterations containing only literal values, which just returns the pkey of the single row for these values (which is usually an integer) instead of a temp table with the same value in each row

bin/map: by_col: Stripping XML functions not in the DB: Remove DB functions based on whether a plain SQL function of that name exists, rather than whether a relational function (i.e. a table) of that name exists. This will allow column-based import to use plain SQL functions that don't have a corresponding relational function.

db_xml.py: Don't remove any explicit pkey because the output table may be a SQL function, which does not have a pkey. This feature only existed to support importing VegBank XML exports, which we don't use (and which would be incompatible with the schema anyway).

sql.py: function_exists(): Fixed bug where select() needed to be run with auto-rollback in case it raised an exception

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.