Values being forward-filled in fixed OPEX table for new entrants

I think that the `forward_fill_values` attribute should be set to `false` for the `fixed_opex_new_entrants` table (but happy to be wrong).

This table has columns relating to open values by regional build cost zone, but rows are by technology. A full row exists for BOTN - Cethana, which should have empty values for all regional build cost zones barring `TAS Medium` column. 