Draft: [python] Remove pybind11 copy

First steps towards removing internal copy of pybind11

• Tests do not yet work.
• Install pybind11 to CI image or as first step install for testing
• Use proper pybind11::header instead of include_directories
• Smooth transition for users and deprecation warnings
• Python tests are failing
• Handle dune/python/pybind11/extensions.h which is not part of pybind11
• Adjust other core modules
  • geometry
  • grid
  • localfunctions
  • istl
• Adjust non-core module
  • dune-uggrid
  • dune-ALUGrid
  • dune-foamgrid
  • dune-polygongrid
  • dune-spgrid
  • dune-functions
  • dune-typetree
  • DuMuX
  • dune-fem
  • dune-fem-dg
  • dune-vem
  • dune-PDELab
  • dune-mmsh
  • dune-vtkwriter