spatialFlux

a subclass can override this, and then modify the spatial

---

Discussion

fluxes that get returned.

To count everything:

1. (u,v,w,eta) -> 4 transformToSpatialDomainWithFourier
2. (u_x,u_y,v_x,v_y,eta_x,eta_y) -> 3 diffX, 3 diffY
3. (Ap,Am,A0) -> 3 transformFomSpatialDomainWithFourier