WFG hyper-volume calculation

This crate contains a Rust wrapper to the C program to calculate the hyper-volume metric using the WFG algorithm proposed by

L. While, L. Bradstreet and L. Barone, "A Fast Way of Calculating Exact Hypervolumes," in IEEE Transactions on Evolutionary Computation, vol. 16, no. 1, pp. 86-95, Feb. 2012, doi: 10.1109/TEVC.2010.2077298.

The original source code is available at https://github.com/lbradstreet/WFG-hypervolume/tree/master under the GPL2 license.

The following changes were applied to the code in the vendor folder (from commit b19f35c):

• the code is now more flexible and can be used a library by calling the calculate_hypervolume function;
• the code was cleanup by removing unnecessary dependencies, formatted and documentation was added;
• the part of the code calculating the hyper-volume from input files was removed.

This code is re-released under the GPL3 license.