exact solution is provided the method returns error=|uh|_0

Also run Ex,Im,Imex in the scalar main code and print the errors of all three
solvers at the end

Merge branch 'bugfix/rkSolver' of ssh://gitlab.dune-project.org:22022/dune-fem/dune-fem-dg into bugfix/rkSolver

RungeKuttaSolver should be physical.

implemented in SpaceOperatorInterface.

Merge branch 'bugfix/rkSolver' of ssh://gitlab.dune-project.org:22022/dune-fem/dune-fem-dg into bugfix/rkSolver

a reasonable test to fail IMEX

Merge branch 'bugfix/rkSolver' of ssh://gitlab.dune-project.org:22022/dune-fem/dune-fem-dg into bugfix/rkSolver

RungeKuttaSolver.

correct operator [].

Cause might have been a wrong keep_alive in the constructor call
but dofs are still invalid in first step in scalar example

a scalar potention but with dimRange=1 it works

some bug needs fixing in elliptic model generation

(see modelwrapper.hh hasDirichletBoundary)