Add more benchmark functions

Since this library is intended for researchers, I think it would be fitting to have a large range of benchmark functions at hand so one does not have to implement them himself. Good sources for such functions are:

• Wikipedia with a solid collection of benchmark functions and a good overview.
• Benchmarkfcns, they also have a quite big collection and even classification into different function categories (might be helpful for the documentation). They have fancy pictures and a short description for every benchmark function, as well as a MATLAB implementation.
• Test Functions Index with a ridiculous amount of benchmark functions. They even classified them by their hardness.

The functions can be implemented in the single_obj.py file. I think a good number to start with is 5 new benchmark functions.

Notes: Please work on the development branch. You can find a good StackOverflow question here.

[whzup]

Welcome @jayspeidell, I’m glad you’re interested! Sure go ahead! 👍 If you have any questions about the project or the GitHub process don’t hesitate to comment on this issue. If you have urgent questions you can also use the gitter chat. Log in using your GitHub account and post your issue 😄.

[jayspeidell]

I can work on this in the next week or so if it’s still open. I looked through the code and project structure and implementing new benchmark functions seems pretty straighforward. I haven’t contributed to any projects before, but I’m learning about particle swarm optimization and this project is interesting.