Add a Python implementation of the mixed Poisson example

This example program does the same as poisson-mfem.cc, but using the Python interface. The only difference is the type of the stiffness matrix. The C++ example uses a 2x2 matrix of BCRSMatrix objects, but that is not easy to replicate in Python unless we start writing Python bindings for dune-istl. This new Python example uses a single flat matrix instead.