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]

Why not the index order (1,3,4,5) ?
Two spaces after colon

[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

[krzysztof]

In this case there's no reason to align variable names in the column with multiple spaces, use a single space.

In this method you're using 8 spaces for indentation. We try to stick with tabs for C++ and 4 spaces for Python code.

[landscape-bot]

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