Number of workers

[VictorXQWang]

It would be nice if one can choose the number of workers. Looks like it is currently defaulted according to the number of CPU cores.

[roedoejet]

The docs say:

"All the parallel p_tqdm functions can be passed the keyword num_cpus to indicate how many CPUs to use. The default is all CPUs. num_cpus can either be an integer to indicate the exact number of CPUs to use or a float to indicate the proportion of CPUs to use."

but I didn't verify