Loads episode_data_index and stats during dataset __init__ (#85)

Co-authored-by: Simon Alibert <75076266+aliberts@users.noreply.github.com>
Co-authored-by: Alexander Soare <alexander.soare159@gmail.com>

tests/data/aloha_sim_insertion_scripted/train/state.json

@@ -4,7 +4,7 @@
       "filename": "data-00000-of-00001.arrow"
     }
   ],
-  "_fingerprint": "d8e4a817b5449498",
+  "_fingerprint": "97c28d4ad1536e4c",
   "_format_columns": null,
   "_format_kwargs": {},
   "_format_type": "torch",