pagmo 2.16.1

This is a minor release which fixes a few build issues recently reported by our users. As a consequence of these fixes, pagmo now requires CMake >= 3.8 when compiling from source.

Note also that, thanks to the recent availability of an Ipopt Windows package in conda-forge, the pagmo conda package for Windows is now built with Ipopt support turned on.

As usual, the full changelog is available here:

https://esa.github.io/pagmo2/changelog.html