Note that the above equations will generate an IndexError where KMT==km,
and are incorrect if partial_bottom_cells=.true..
To handle pbc grids, I suggest adding bottom_cell_file to pop_grid_definitions.yaml.

From @klindsay28 in #44 (comment)

The following (untested) code replaces nested j,i loops with a single k loop.
It seems like it would be more cache friendly.

for k in range(VOL.shape[0]):
    VOL[k, :, :] = np.where(k < KMT, VOL[k, :, :], 0.0)

We'll need this to specify VOL as an xgcm metric.