Fix depth slicing with non-ocean cells

Previously, when there were cells with `maxLevelCell = 0`, this
caused a failure because there was no valid depth coordinate
for that cell and therefore no closest depth index could be found.

This fix simply skips cells that are not in the ocean and masks
them out.

mpas_analysis/ocean/remap_depth_slices_subtask.py

@@ -116,6 +116,10 @@ def run_task(self):
 
         zMid = compute_zmid(ds.bottomDepth, ds.maxLevelCell-1,
                             ds.layerThickness)
+        ocean_mask = (ds.maxLevelCell > 0)
+
+        print(f'zMid min: {zMid.min().values}, max: {zMid.max().values}')
+        print(f'maxLevelCell min: {ds.maxLevelCell.min().values}')
 
         nVertLevels = zMid.shape[1]
         zMid.coords['verticalIndex'] = \
@@ -135,6 +139,8 @@ def run_task(self):
 
         for depthIndex, depth in enumerate(self.depths):
             depth = self.depths[depthIndex]
+            print(f'depth: {depth}')
+
             if depth == 'top':
                 # switch to zero-based index
                 verticalIndices[depthIndex, :] = 0
@@ -144,10 +150,12 @@ def run_task(self):
                 verticalIndices[depthIndex, :] = self.maxLevelCell.values
                 mask[depthIndex, :] = self.maxLevelCell.values >= 0
             else:
+
+                diff = np.abs(zMid - depth).where(ocean_mask, drop=True)
+                verticalIndex = diff.argmin(dim='nVertLevels')
 
-                verticalIndex = np.abs(zMid - depth).argmin(dim='nVertLevels')
-
-                verticalIndices[depthIndex, :] = verticalIndex.values
+                verticalIndices[depthIndex, ocean_mask.values] = \
+                    verticalIndex.values
                 mask[depthIndex, :] = np.logical_and(depth <= zTop,
                                                      depth >= zBot).values