csv2db: Adding indexes: Fixed bug where sql.add_index()'s ensure_not_null param needed to be renamed to ensure_not_null_

sql.py: cast(): columns values clause: Use start=0 to avoid "SELECT statement missing a WHERE, LIMIT, or OFFSET clause" warning

inputs/import.stats.xls: Updated for most recent run

sql.py: Removed no longer needed mk_track_data_error()

sql.py: track_data_error(): Use for loop and insert() (ignoring DuplicateKeyException) to insert entries into the errors table, to get the same optimization benefits this change provides in other filter-out contexts, and to improve clarity

sql.py: cast(): Use FOR loop with EXCEPTION block instead of CROSS JOIN with LEFT JOIN to insert entries into the errors table, to get the same optimization benefits this change provides in other filter-out contexts, and to improve clarity

sql_gen.py: NamedValues: Support None cols param for no named columns

sql.py: ensure_not_null(): Made the use of index columns configurable based on the # of rows in the table, because for small datasources, they seem to add 6-25% to the total import time

xml_func.py: _noCV: Fixed bug where assumed items was an iterator when it's now a list

sql.py: add_index_col(), cast_temp_col(): Cache add_col() by providing new comment param to distinguish new columns of the same (colliding) suffixed name but from different source columns