Skip parser matching when parser is already specified

From a discussion with @ladinesa we came with the following proposal: atm, running nomad parse --parser <parser name> --show-archive will still perform the parser matching. We know this, since some auxiliary files (e.g. OUTCAR) are not being picked up on in the presence of the main file (e.g. vasprun.xml). nomad then just returns assert parser is not None, 'there is no parser matching %s' % mainfile. I'd argue that this behavior is counterintuitive in light of the --parser <parser name> option.

If we change this, I'd also request to add further specification in nomad parse --help on how to formulate <parser name>. Trial-and-error doesn't work in guessing the naming format, and I eventually only figured it out thx to @ladinesa . If the naming can be customized via a settings file, just pointing to its location is also fine.

@mscheidg I'd really like your feedback, given that this touches on the command line behavior.