removed unused import statement; fixed return bug and added exception…

… case for fill_cells~_vertically()

mfsetup/discretization.py

@@ -256,6 +256,11 @@ def fill_cells_vertically(top, botm):
     -------
     top, botm : filled top and botm arrays
     """
+    # check for nans in "top" array, if any nans exist, raise an error
+    # nans in top array could have unexpected effects
+    if np.isnan(top).any():
+        raise ValueError("Error: The top array contains NaN values. Ensure these are defined to prevent unintended results.")
+
     thickness = get_layer_thicknesses(top, botm)
     assert np.all(np.isnan(thickness[np.isnan(thickness)]))
     thickness[np.isnan(thickness)] = 0
@@ -271,7 +276,7 @@ def fill_cells_vertically(top, botm):
     filled += np.nanmin(all_surfaces, axis=0)  # botm[-1]
     # append the model bottom elevations
     filled = np.append(filled, [np.nanmin(all_surfaces, axis=0)], axis=0)
-    return filled[1:].copy()
+    return top, filled[1:].copy()
 
 
 def fix_model_layer_conflicts(top_array, botm_array,