Remove the `ironing out iteration 0` prefix from debug files when the ironing out loop is turned off

Minor niggle, but when the ironing out loop is turned off (i.e. the default behaviour), I find it annoying how all the `run_description` rows in some of the files are prefixed with "ironing out iteration 0". I think we only need to include this prefix when the ironing out loop is turned on (i.e. more than one iteration).