fixes required when using `-s`/`--select` option with `mdb launch`

[TomMelt]

Two issues when using the -s/--select option with mdb launch.

1. unselected ranks adds unnecessary -- before self.args which stops them working. https://github.com/TomMelt/mdb/blob/f91d9820b2cbf19d2e8762383e40130f8d3cfa68/src/mdb/mdb_wrapper.py#L187
2. stdout is only redirected for selected ranks https://github.com/TomMelt/mdb/blob/f91d9820b2cbf19d2e8762383e40130f8d3cfa68/src/mdb/mdb_wrapper.py#L176-L180