Allow users to set number of process pool workers/threads

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.