Add first test for slicing

This MR will allow to slice OfflineTracks, so they return OfflineTracks again (for consistent behaviour). Currently an OfflineTrack is returned, regardless of the slice type (tracks[23] vs tracks[5:23]).

tests/test_offline.py

@@ -306,6 +306,12 @@ class TestOfflineTracks(unittest.TestCase):
         self.assertListEqual([0.230189, 0.230189, 0.218663],
                              list(dir_z[0][:3]))
 
+    def test_slicing(self):
+        tracks = self.tracks
+        assert 10 == len(tracks)
+        track_selection = tracks[2:4]
+        assert 3 == len(track_selection)
+
 
 class TestOfflineTrack(unittest.TestCase):
     def setUp(self):