Ability to kill run mid-dispersion and not lose what's already been processed

[jdubowy]

• [ ] come up with way to signal to bluesky, mid-run, to abort dispersion
• [ ] combine hysplit .conc files from tranches (preferably creating a single .conc that then gets converted to netcdf, as the code already does, but possibly by converting each to netcdfs which are then combined into a single netcdf.