Pad calc_dirs with zeros

[jlnav]

Funnily enough to do this, we have to send exit_criteria to the workers.

[shuds13]

My understanding of this was something like:

libE_specs["simid_field_width"] = 5.

ensemble/
      sim00001
      sim00002
      sim00003`

If an H0 is supplied, another sim_max calculations will be done, so it could produce more directories. Also, may end based on a value, rather than sim/gen_max.

[shuds13]

Maybe a unit test.

[shuds13]

and update the top comment here.

[shuds13]

I ran

python test_GPU_variable_resources.py --comms local --nworkers 4

and it gave me a width of four, without adding the option.

[shuds13]

This works well, and adds digits on overflow as needed.

I think we just need to check we agree on the option name "calc_dir_id_width".