esa/pagmo2: pagmo 2.16.0

This new release focuses mostly on polish and bug fixes. Specifically: a couple of important issues have been rectified in the PSO and genetic algorithms, adding individuals to a population is now substantially faster, various build issues affecting newer versions of pagmo's dependencies have been corrected. Other changes: pagmo now requires a C++17-capable compiler, the pagmo library can now be built also a static library, pagmo can now be built with link-time optimisations enabled. The full changelog, as usual, is available here: https://esa.github.io/pagmo2/changelog.htm