ZMCintegral-importance sampling and hueristic tree search

This version provides a better for multi-gpu calculation. Besides, user can adjust chunk_size_x to have better performance for specific gpu apparatus.
Also, different method is supported here.

ZMCintegral-importance sampling and hueristic tree search

This version provides a better for multi-gpu calculation. Besides, user can adjust chunk_size_x to have better performance for specific gpu apparatus.

ZMCintegral-importance sampling and hueristic tree search

This version adds the function of importance sampling. To make the integration more suitable to very deep and high peak integrands, a heuristic tree search method is applied so that the domain can be amplified again and again.

ZMCintegral-only random sampling

This version only accomplishes the random sampling.