We are trying to simplify the logic in program `inst_eddies`. Also,
the program `inst_eddies` should not write `grid_nml.txt` to the top
level of the SHPC directory since several instances could run in
parallel for several slices. So we are leaving it as a task for a
script above `inst_eddies` to create `grid_nml.txt` at the top level
of the SHPC directory. In tests, we have to move `grid_nml.txt` to
SHPC because we use `SHPC/grid_nml.txt` in other tests.