add python script for mmdg convergence test

dune/mmdg/mmdg.hh

src/CMakeLists.txt

src/dgAnalysis.py

src/grids/mmdg2_1.msh

src/grids/mmdg2_2e-1.msh

src/grids/mmdg2_5e-1.msh

src/grids/mmdg2_5e-2.msh

src/grids/mmdg2_8e-2.msh

src/mmdg.cc

src/mmdgAnalysis.py

src/parameterMMDG.ini