How to interpret the process density progress bar

[sjteresi]

@teresi Can you please explain how to interpret the TE density progress bar numbers? For example when it is calculating density it will say subsets: show a progress bar and then display a fraction to the right of the progress bar indicating how many subsets were calculated.

How do I interpret this fraction? I looked to merge_data.py and _list_density_args() and _list_sum_input_outputs() and was unable to determine what that number represents.

I would like to add this information to the documentation/README for users.