improve the flux class setup a bit

dune/fem-dg/misc/default.hh

dune/fem-dg/operator/fluxes/advection/python.hh

pydemo/adrsolver/scalar.py

pydemo/chemicalreaction/main.py

pydemo/chemicalreaction/micap.py

pydemo/chemicalreaction/micap1.py

pydemo/chemicalreaction/paper.py

pydemo/chemicalreaction/test.py

pydemo/euler/euler.py

pydemo/euler/llf.hh

pydemo/euler/paper.py

pydemo/rk/euler.py

pydemo/scalar/scalar.py

pydemo/shallowWater/main.py

pydemo/shallowWater/paramBase

pydemo/shallowWater/paramSolver

pydemo/shallowWater/parameter

pydemo/shallowWater/shallowwater.py

python/dune/femdg/_operators.py

python/dune/femdg/patch.py