cleanup index2volume of HPRadialGridAtLevel

src/re/multi_grid/grid_impl.py

@@ -366,11 +366,10 @@ class HPRadialGridAtLevel(MGridAtLevel):
         return super().coord2index(coord, **kwargs)
 
     def index2volume(self, index):
-        ndims_off = tuple(np.cumsum(tuple(g.ndim for g in self.grids)))
-        islice = tuple(slice(l, r) for l, r in zip((0,) + ndims_off[:-1], ndims_off))
-        r_upper = self.grids[1].index2coord(index[islice[1]] + 0.5)
-        r_lower = self.grids[1].index2coord(index[islice[1]] - 0.5)
-        A_unity = self.grids[0].index2volume(index[islice[0]])
+        grid_hp, grid_r = self.grids
+        r_upper = grid_r.index2coord(index[1:2] + 0.5)
+        r_lower = grid_r.index2coord(index[1:2] - 0.5)
+        A_unity = grid_hp.index2volume(index[0:1])
         return A_unity * (r_upper**3 - r_lower**3) / 3