Pybind11 v2
Adapt to pytbind11 v2 (almost). From commit message: updated to new pybind11: this is not a released version but the master version tag 5e92b3e608c98e1484a8a237db4b03bd807452a3 Reason is a problem also discussed in issue 608 https://github.com/pybind/pybind11/issues/608 which is not fixed in the release.
In addition a method "polygons" is added to the grid