Convergence of Non-Standard FDTD Solver

[anekslen]

The Non-Standard FDTD solver only converges for a very small numer of grid points and the does not converge anymore when increasing the number of gridpoints. This can be seen in the image below.

The convergance should be better, the reason behind this needs to be investigated.
The convergence is tested with the TestNonStandardFDTDSolver_convergence test in test/solver/.