user defined worker count for e3sm_diags

[sterlingbaldwin]

Right now the num_workers is set to 24, this should be an option users can set for themselves.

[zshaheen]

By the way, for e3sm_diags, if a person doesn't pass in a num_workers argument but has multiprocessing=True, the number of workers defaults to 4.

[sterlingbaldwin]

The template I use for the parameter file has num_workers = 24. This should be replaced with an option to allow the user to define it or leave it blank, in which case it would revert to the default of 4.