with ice shelf cavities Depth.data is no longer seafloor depth but distance from seafloor to sea surface

[ifenty]

I expected the model grid output field Depth.data to be seafloor depth, but in the V4r5 output the values of Depth.data`` are now the distance between the seafloor and the sea surface. In open water areas, this is expected but where we have ice-shelf cavities, Depth.data`` is the distance between the seafloor and the ceiling of the ice shelf cavity. We need to make it seafloor depth again to correspond with its name. @owang01