uu = (1-ddx)*(1-ddy)*u11 + ddx*(1-ddy)*u21 + ddx*ddy*u22
. + (1-ddx)*ddy*u12
_RL dx, dy, dz, ddx, ddy, ddz
ddx = dx
ddx = dx + 0.5
ddx = dx - 0.5
uu = (1-ddx)*(1-ddy)*(1-ddz)*u111 + ddx*(1-ddy)*(1-ddz)*u211
& + ddx*ddy*(1-ddz)*u221 + (1-ddx)*ddy*(1-ddz)*u121
& + (1-ddx)*(1-ddy)*ddz*u112 + ddx*(1-ddy)*ddz*u212
& + ddx*ddy*ddz*u222 + (1-ddx)*ddy*ddz*u122
_RL dx, dy, ddx, ddy