Api update

merge request so that you can comment the code ;)

km3io/aanet.py

@@ -200,7 +200,7 @@ class AanetReader(AanetKeys):
         if self._mc_tracks is None:
             self._mc_tracks = AanetTracks(
                 self.mc_tracks_keys,
-                [self._lazy_data[key] for key in self.mc_tracks_keys])
+                [self._data[key] for key in self.mc_tracks_keys])
         return self._mc_tracks
 
 
@@ -261,17 +261,17 @@ class AanetHits:
         return AanetHit(self._keys, [v[item] for v in self._values])
 
     def __len__(self):
-        return len(
-            self._values[0]
-        )  # I don't like this being explicit, what if values is empty ...
-
+        try:
+            return len(self._values[0])
+        except IndexError:
+            return 0
     def __str__(self):
         # hits
         if all(key.startswith('hits.') for key in self._keys):
             return "Number of hits: {}".format(len(self))
         # mc hits
         if all(key.startswith('mc_hits.') for key in self._keys):
-            return "Number of mc hits in the selected event: {}".format(
+            return "Number of MC hits: {}".format(
                 len(self))
 
     def __repr__(self):