Allow users to set number of process pool workers/threads

[windowshopr]

Would be nice to see the users be able to control the number of processors/threads used when defining 'multiprocessing' or 'multithreading'.

I would think you could pass in a worker argument (default can be None) to set_run_mode(), something like:

set_run_mode(obj_func, 'multiprocessing', n_workers=3)

..and then further down in the func_transformer() function:

    elif mode == 'multiprocessing':
        from multiprocessing import Pool

        pool = Pool()
        if n_workers != None:
            pool = Pool(n_workers)

        def func_transformed(X):
            return np.array(pool.map(func, X))

I'm not sure if this would be the appropriate or working syntax, but just an idea I had. Sometimes we don't want to just default to using ALL CPU cores when using multiprocessing.