There seem to be somehow multiple py::kwargs classes created in
different libraries, because of this the any_cast is failing.
To circumvent this we attach them to the MeshingParameters object
as flags.
Forward-declare pybind11::object and move implementation of
Archive::Shallow() to new header python_ngcore.hpp
All files using the Shallow/Python archive functionality must include
core/python_ngcore.hpp. Missing includes result in link errors, due to
missing instantiations of Archive::Shallow<T>();
