outsource reconstruction data in separate arrays

Previously, all fit information data (corresponding key: 'fitinf') from offline files was given to the user in a big chuncked array. To address the issue #16 (closed) 'fitinf data' is restructured into a numpy rec array.

km3io/offline.py

@@ -537,7 +537,7 @@ class OfflineReader:
 
             yield trk_index, stages_index
 
-    def _find_empty(self, chunk_arr):
+    def _find_empty(self, array):
         """finds empty lists/arrays in a chunked-jagged array of data
         of interest.