Unhide counter from mc_trks

In https://git.km3net.de/common/km3net-dataformat/-/issues/93 the counter field was added to Trk which is used in MC tracks. This needs to be unhidden from mc_tracks, which is done in this MR.

tests/test_offline.py

@@ -490,11 +490,13 @@ class TestOfflineTracks(unittest.TestCase):
             self.f.events[3:5][1].tracks.fitinf[9][2],
         )
 
+
 class TestMisc(unittest.TestCase):
     def test_mc_tracks_counter(self):
         r = OfflineReader(data_path("gseagen/DAT000001.gSeaGen.1.aa.root"))
-        np.testing.assert_allclose([1,2,3], r.mc_tracks.counter[0])
-        
+        np.testing.assert_allclose(
+            [0, 0, 6, 7, 0, 0, 2, 53, 0, 0, 6, 57, 0], r.mc_tracks.counter[0][:13]
+        )
 
 
 class TestBranchIndexingMagic(unittest.TestCase):