add 'default' in `--help`

In the current scripts, for example `/py/desispec/scripts/preproc.py`, `desi_preproc --help` does not give the default values. This can be easily fixed by adding `formatter_class=argparse.ArgumentDefaultsHelpFormatter,` in each `argparse.ArgumentParser(`.
This allows not to have to dig into the code to get some useful information.

Start with /py/desispec/scripts/preproc.py and inspect the argparse.ArgumentParser calls in the current scripts. Run desi_preproc --help before and after the change to verify that each help output includes argument default values; done means the affected scripts consistently use the requested formatter.

Written by the indexing model from the issue text.