Fix weno to work with standard examples

Currently working examples

• advection + sine
• advection + bump
• burgers + sine
• burgers + gaussian
• euler + smooth
• euler + sod shocktube
• sr euler + sod shocktube
• sr euler + blast wave 1

Blast wave 2 test is not working, but none of the other methods is working in this case either. The overall quality of the weno implementation is also not good. Although I think the weno implementation together with the splitting technique is correct, the results are still very poor. But I don't see where the problem is ...