add functionality to seisflows clean command

[bch0w]

Often seisflows users will want to keep their working directory for reference, or to hold onto results, but they will no longer be running workflows in it. Therefore, lots of the scratch files are unnecessary, but large and taking up space on the machine. It would be useful to have a variation of the clean command that scraps these large files, ensures that everything worth keeping is in the output directory, and maybe sets up the working directory so that it could be used again without too much manual work.

I have tried to do this previously with personal scripts but haven't done it generally. This would be good practice as I know many users have many seisflows working directories and if their SPECFEM models are large this becomes a storage problem.

https://github.com/bch0w/simutils/blob/master/seisflows/debug_tools/clean_scratch.py