Add the type constraint orientation::Integer to every flux function

[NichtLucas]

As discussed here, we should probably add the type constraint orientation::Integer to every flux function to distinguish normal vectors and orientations.

[ranocha]

The second part of this issue is to (possibly?) get rid of FluxRotated and create some syntactic sugar to generate the necessary methods specialized on normal::AbstractVector instead of orientation::Integer. A simple macro should do the job.