Split TimeStepSizeComputation and merge parts with SolutionUpdate and LocalRecomputation

• TimeStepSizeComputation will be reduced to a simple time step size computation function. SolutionUpdate and LocalRecomputation will also compute a time step size and will further advance in time. This will reduce logic.

• Next step will be a fusion of multiple algorithmic phases of the ADER-DG and Limiting ADER-DG schemes in a single solver function. This will hopefully make it easier to optimise for the compiler and easier for the processor cache to decide what to hold or drop.

• We might be able to get rid of mapping FusedTimeSteppingInitialisation with the above split.