Save and load config file (format) for decoder

[lenzo-ka]

It would be a nice piece of sugar to be able to load and write configurations from the API (and/or cli args). This would make it easy to re-run things with the same arguments (replicating results and usinga setup on new items), and also open it up to editing the config file to change startup params. Could also potentially contain adapted cmn parameters.

[lenzo-ka]

It would be super to have a verbose save that actually includes the values for all parameters, rather than just the ones that are "set" (or differ from defaults).

[dhdaines]

Yes, this is a good idea and I will do it. Possibly just the "argfile" format that is already supported (but not for saving) even though something like YAML would obviously be a lot nicer.

[dhdaines]

Implemented in #295 with ps_config_serialize_json() - may need a bit of tweaking, currently it is not saving string parameters that are NULL but I think it should save them since that might not be the default value