Argument to set n_threads to a value smaller than os.count_cpu()

[flcvlr]

Congratulations for the great software.

I was wondering whether it is possible to introduce an option to change the default number of threads for parallel tasks (in my case EM function during RNA segmentation, but this may apply to other functions, probably).

This could be useful, in my case, as I am running RNA segmentation on smaller areas ( < 1 square mm) and then joining these "tiles" together. Since most of the time of RNA segmentation (~60% in my case) is spent in single-core operations, a more efficient parallelization could be attained by launching the segmentation script in parallel for several "tiles", say 4 or 6, reserving 6 or 8 threads for each. In the current setting this is impossible as each segmentation will use all cores, but for very short intervals of time.

Thanks again for making available this great tool!

Valerio