sql.py: cast(): Made errors table also store SQLSTATE in error_code column

sql.py: cast(): Documented that the value and error are inserted for each source column (hence the CROSS JOIN)

sql.py: cast(): Version the function name if using an errors table, to avoid collisions with other cast functions when the function name is truncated (or, more rarely, collisions with casts to the same type and on the same input columns but of a different table)

sql.py: cast(): function_name: Fixed bug where sql_gen.FunctionCall()'s parameters needed to be passed with *args syntax

sql.py: mk_flatten_mapping(): Propagate the original columns' sources to the flattened columns so they won't be lost in the flattening

sql.py: put_table(): MissingCastException: Use in_tables0's source table to locate the errors table in case in_tables0 has been subset into a temp table (which removes the schema name)

db_xml.py: put_table(): Track in_table's source so its original schema can be obtained and auxiliary tables located

sql_gen.py: Derived.set_srcs(): Added optional overwrite param so that a default srcs value can be set only if one isn't already set

sql_gen.py: Made Table a Derived element so that a row-subset temp table could retain the schema of the table it came from, and any auxiliary tables in that schema could be located properly

sql_gen.py: Moved srcs-related functionality from Col to new superclass Derived