Add the `Ignore` field, which allows fields in a file to be accounted for in the validation, but skipped entirely. This is particularly useful for tables where certain fields have been pre-populated with data before the rows are completed (e.g. adding row numbers for 3000 rows when you only have 5 rows of data so far). This additionally allows the row check to use the `Ignore` class when determining if the row should be considered empty (for the purpose the `empty_row_ok` conditional evaluation).