esa/pagmo2: pagmo 2.15.0

This new pagmo release features various enhancements to the topology API, and additional runtime introspection capabilities for all type-erased classes. It also fixes a serious issue appearing on certain compiler/platform combinations where dynamic_cast does not work across dlopen()ed shared libraries. Another important fix involves compilation with recent CMake versions. The full changelog, as usual, is available here: https://esa.github.io/pagmo2/changelog.html