Validate ThreadContext.num_workers

[cydanil]

Hi @philsmt,

Here is a possible improvement validating the given ThreadContext.num_workers at initialization.

Messing around with AGIPD notebooks, I noticed that the notebook may assign 0 threads if there's more files than cores availables.

It's most likely a non-issue but it's a slow shift it's a simple validation to add here.

I've tested the changes as seen in the test file.

Thanks!
Cyril