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.

NW (-1/2, 1/2) and the like were previously mapped to the wrong cell

- str(e) did work in previous pytest version
- but the correct usage is str(e.value)
- in newer pytest version the string for the e object is changed so
  the tests did not work any more

This restructuring allows for easier separation of modules into
separate repositories later. Also, now pip install with repo url can be
used.

The setup.py files have also been updated to correctly reference each
other. Module versions are not extracted from git state