Tutorial for the dune-fem python bindings

A detailed description of the dune-fem python bindings is now available. It contains an introduction to the main concepts and to the core bindings. Many individual Python script / Jupyter notebooks are available for download showing how to solve a wide range of complex problems. These provide a good starting point for new users. Extensions of DUNE-FEM (e.g. solvers based on a wide range of Discontinuous Galerkin and Virtual Element methods) are also showcased.