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

@@ -543,7 +543,7 @@ class OfflineReader:
 
         Parameters
         ----------
-        chunk_arr : Chunked array
+        array : awkward array
             Chunked array or jagged array of data of interest. For example:
             km3io.OfflineReader(my_file).tracks.fitinf .