stableTimeStepSize kernels should return maximum eigenvalue for (classical) local time stepping

In general, we cannot associate the smallest time step size with the finest mesh level as there might be larger eigenvalues present on coarser mesh levels.

This is an issue for local time stepping where you scale the smallest time step size by a factor k (k=3 in Peano) with decreasing mesh level.

The kernels should thus return the maximum eigenvalue, too, or only the maximum eigenvalue.

The minimum local time step size would then be computed according to:

dt_min = CFL * max_{over all cells} lambda / min_{over all cells} cellSize 

which is different to what we are currently doing for global time stepping:

dt_min = CFL * min_{over all cells} ( lambda / cellSize ) 

The ADERDGSolver superclass would then decide which minimisation to perform.