Adding support for grid.negate_depth() method

This is useful when the depth needs to be flipped from positive to negative values. Note that this does _not_ change the direction of the vertical velocity; for that users need to add a fieldset.W.set_scaling_factor(-1.0)

parcels/grid.py

@@ -114,6 +114,9 @@ def lat(self):
     def depth(self):
         return self._depth
 
+    def negate_depth(self):
+        self._depth = -self._depth
+
     @property
     def mesh(self):
         return self._mesh

tests/test_grids.py

@@ -112,6 +112,16 @@ def test_time_format_in_grid():
         RectilinearZGrid(lon, lat, time=time)
 
 
+def test_negate_depth():
+    depth = np.linspace(0, 5, 10, dtype=np.float32)
+    fieldset = FieldSet.from_data(
+        {"U": np.zeros((10, 1, 1)), "V": np.zeros((10, 1, 1))}, {"lon": [0], "lat": [0], "depth": depth}
+    )
+    assert np.all(fieldset.gridset.grids[0].depth == depth)
+    fieldset.U.grid.negate_depth()
+    assert np.all(fieldset.gridset.grids[0].depth == -depth)
+
+
 def test_avoid_repeated_grids():
     lon_g0 = np.linspace(0, 1000, 11, dtype=np.float32)
     lat_g0 = np.linspace(0, 1000, 11, dtype=np.float32)