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.

   mdb/src/mdb/mdb_wrapper.py

   Line 187 in f91d982

   line = f"-n 1 {self.target} -- {self.args}"

2. stdout is only redirected for selected ranks

   mdb/src/mdb/mdb_wrapper.py

   Lines 176 to 180 in f91d982

   	if self.redirect_stdout is not None:
   	options = options + [
   	"--redirect-stdout",
   	f"{self.redirect_stdout}",
   	]