Add argument to choose number of threads used in multiprocess

[PierreLaville]

• Why? : because of an excessive amount of RAM used (large files loaded?) which leads to stop computation (Killed)
• Solution (doesn't solve the cause): limit the number of threads used in multiprocess (default is all of them)
• Example: https://github.com/huguesrichard/Allopipe/blob/c735c7559a709ab84dcc12598092489e56c4b43e/src/multiprocess_ams.py#L97

Replace with: with concurrent.futures.ProcessPoolExecutor(max_workers=12) as executor: for 12 threads (use variable with argument value instead)

[huguesrichard]

I think this is a very good idea. Would there be a way to read the amount of possible threads and fix it to half of it by default?