move from _ff_cross_references.py to here, remove imports from 29

#: Codes used in STASH_TO_GRID which indicate the x coordinate is on the
#: edge of the cell.
X_COORD_U_GRID = (18,)
#: Codes used in STASH_TO_GRID which indicate the y coordinate is on the
#: edge of the cell.
Y_COORD_V_GRID = (19,)

HANDLED_GRIDS = (1, 2, 3, 4, 5) + X_COORD_U_GRID + Y_COORD_V_GRID