Pass FluxVarsCache to Neumann Function
We build the flux variable caches for boundary faces, but do not hand them into the Neumann function, where it could be used. For box, we actually never use the object, so it's unnecessary overhead.
We build the flux variable caches for boundary faces, but do not hand them into the Neumann function, where it could be used. For box, we actually never use the object, so it's unnecessary overhead.