Fast Bounded Power Diagram

Optimised version of bounded power diagram for large number of sites

The code obtains power diagram of sites bounded with a rectangular or a square bounding box.
This is the optimised version of power diagram for large number of sites (e.g. 10000 or more). In my computer, it finishes the function with 1 million sites within 16s. An empirical test shows that the running time is O(N^(2/3)) with N is the number of sites.