More staggered grid improvements

• Fix access to directions with mixed signs (NW, (-1/2, 1/2) and the like). They were previously mapped to the wrong cell.
• When storing fluxes on a staggered grid, the usual sign convention is that fluxes point outward from the cell. Previously, we did not consistently respect that (staggered_access("E") would return the same thing as staggered_access("W") did 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.
• staggered_vector_access returns a vector/tensor full of Access objects corresponding to the shape (index_shape[1:]) of the staggered field