[FEATURE] Specify number of cores used for datacube generation

[adamjtaylor]

Is your feature request related to a problem? Please describe.
Datacube generation is parallelised for speed. By default, it uses all available threads which may compromise the performance of other processes being run by the user.

Describe the solution you'd like
Enable the user to specify the number of cores used for datacube generation (initially in command line usage but might be nice to have in the GUI as well). Suggest changing the default to number of cores/threads minus 1 to leave one free for other user processes.

Describe alternatives you've considered
This is likely already implemented in MATLAB preferences and we just need to call a helper function. We could even just suggest in the docs where the user can adjust themselves.

Additional context
Chugging away nicely on 20 threads.

[adamjtaylor]

I think we can control this with maxNumCompThreads

max_threads= maxNumCompThreads; maxNumCompThreads(max_threads-1);