Add hexagonal topology to pso

[sriki18]

This will add a hexagonal topology to pso. (I've fixed things that were wrong or missing in the previous PR)

[krzysztof]

I can't judge just by the code whether it's implemented correctly (do you have a reference implementation?). This needs some tests (ideally both C++ and Python) to verify that it works as reported in the literature. Do you have any data on it ?

Update Python docstring: https://github.com/esa/pagmo/blob/master/PyGMO/algorithm/init.py#L264-L268

[landscape-bot]

Code quality remained the same when pulling 607d523 on sriki18:master into 3d4c309 on esa:master.