[ENH] using spawn with process pool instead of fork

[HYChou0515]

Instead of using the default spawn of multiprocessing, we use fork to solve issue described this SO.

# we are doing it now
if platform == 'darwin':
    set_start_method("fork")

However, using fork may cause memory leak issue and segmentation fault ( #21 ). It seems more preferable using process pool and we don’t need to use fork.