Fix musclhancock scheme for certain patch sizes

For me, for certain patch sizes, our 2nd order FV scheme in ExaHyPE (musclhancock) fails. Godunov runs fine. This has to be debugged (it certainly depends on the patch size: Some work, some not).

2nd order is crucial for some applications (GRMHD,CCZ4).

I will do this, ticket just for book keeping.