staggered_access: optionally reverse sign for access via opposite direction
When storing fluxes on a staggered grid, the usual sign convention is that fluxes point outward from the cell. Previously, we did not respect that as staggered_access("E") would return the same thing as staggered_access("W") would when called from the eastern-next cell. Now, when a field is declared as STAGGERED_FLUX, it returns an accessor with a prefactor of -1 in that case. The previous behavior where sign is not reversed is still useful when e.g. storing sums (e.g. mean values) instead of differenes (e.g. finite difference fluxes) on the staggered grid.