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]).

km3io/offline.py

@@ -570,8 +570,15 @@ class OfflineTracks:
             setattr(self, k, v)
 
     def __getitem__(self, item):
-        return OfflineTrack(self._keys, [v[item] for v in self._values],
-                            fit_keys=self._fit_keys)
+        if isinstance(item, int):
+            return OfflineTrack(self._keys, [v[item] for v in self._values],
+                                fit_keys=self._fit_keys)
+        else:
+            return OfflineTracks(
+                self._keys,
+                [v[item] for v in self._values],
+                fit_keys=self._fit_keys
+            )
 
     def __len__(self):
         try: