Test ascending (#302)

* New test for ascending values

* second commit with a last blank line

* Add missing import

---------

Co-authored-by: Daniele Nerini <daniele.nerini@gmail.com>

pysteps/tests/test_extrapolation_semilagrangian.py

@@ -1,5 +1,6 @@
 # -*- coding: utf-8 -*-
 import numpy as np
+import pytest
 from numpy.testing import assert_array_almost_equal
 
 from pysteps.extrapolation.semilagrangian import extrapolate
@@ -23,6 +24,16 @@ def test_semilagrangian():
     assert_array_almost_equal(result, expected)
 
 
+def test_ascending_time_step():
+    precip = np.ones((8, 8))
+    v = np.ones((8, 8))
+    velocity = np.stack([v, v])
+
+    not_ascending_timesteps = [1, 2, 3, 5, 4, 6, 7]
+    with pytest.raises(ValueError):
+        extrapolate(precip, velocity, not_ascending_timesteps)
+
+
 def test_semilagrangian_timesteps():
     """Test semilagrangian extrapolation with list of timesteps."""
     # inputs