changing limiter from minmod to koren

I am not sure if overwriting the MINMOD limiter by a Koren Limiter is a good idea for all applications.

We should rather provide a mechanism for switching between different limiters.

I agree. Which is why we have it as a separate branch. In fact, we wanted to ask you what the best approach for this would be.

Thanks for the comment, @domcha. I think the way to go is to implement the flux limiter as a choice in the FV solver section in the toolkit, and to call a solver method right in the kernels::finitevolumes::musclhancock::c::solutionUpdate in place of the former minmod function. Choices would be then: minmod, koren, user-defined, where the latter creates a function stub in the generated user FV solver. Toolkit code generation as usual.

I will implement this in a seperate feature branch and merge it then into various branches (such as master, as well as Lukes current working branch, i.e. the minmod branch).

One principal thing is that this function is scalar, while it probably should be vectorized. That is easily possible with inline functions, but not any more with user-defined ones. But I don't think there is an urgent need for user-defined anyway. Maybe we can just offer a choice. Adding another one should then be an easy task.

Since Luke now uses the master branch, I implemented these features in the toolkit in 6ddca16a in the master branch, independently from this branch (and the associated one-month-old fork from master).