Feature/materiallaw 2p course

Fixing the materiallaw for all 2p excercises.

• There is the compile warning variable ‘hasNewAtPos’ set but not used [-Wunused-but-set-variable] in the excercise-fracture when using the function:

template<class ElementSolution>
auto fluidMatrixInteraction(const Element& element,
                            const SubControlVolume& scv,
                            const ElementSolution& elemSol) const

• In the solution/excercise-fracture there are 2 warnings: warning: when initialized here [-Wreorder] which I could'nt resolve.
• Issue #27 (closed)