cleanup/staggeredupwindfluxvars ifconstexpr

While looking through the freeflow implementation again I noticed that the private functions in the staggeredupwindfluxvariables.hh header could be reduced using c++17 features. This would reduce the code a good amount, and hopefully make it easier to read through.

The commits are hopefully split up so that the review isn't too difficult. They can be squashed if need be. This is very low priority; it's just clean up.