Best reco hits events

This addresses issues #22 (closed) and #24 (closed)

This is ready for merge into master. Although I am not sure I understand why the code style fails here on git (I updated yapf on my computer and used it to correct the code style but maybe I am missing a detail here?).

km3io/offline.py

@@ -313,7 +313,7 @@ class Reader:
             do not contain data. Therefore, the keys corresponding to these
             fake branches are not read.
         """
-        keys = set(self.keys.valid_keys)
+        keys = self.keys.valid_keys
         if key not in keys and not isinstance(key, int):
             raise KeyError(
                 "'{}' is not a valid key or is a fake branch.".format(key))
@@ -334,7+334,7 @@
         Class
             OfflineKeys.
         """
         if self._keys is None:
             self._keys = OfflineKeys(self._file_path)
         return self._keys