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/xarm_push_medium/train/dataset_info.json

@@ -0,0 +1,51 @@
+{
+  "citation": "",
+  "description": "",
+  "features": {
+    "observation.image": {
+      "_type": "Image"
+    },
+    "observation.state": {
+      "feature": {
+        "dtype": "float32",
+        "_type": "Value"
+      },
+      "length": 4,
+      "_type": "Sequence"
+    },
+    "action": {
+      "feature": {
+        "dtype": "float32",
+        "_type": "Value"
+      },
+      "length": 3,
+      "_type": "Sequence"
+    },
+    "episode_index": {
+      "dtype": "int64",
+      "_type": "Value"
+    },
+    "frame_index": {
+      "dtype": "int64",
+      "_type": "Value"
+    },
+    "timestamp": {
+      "dtype": "float32",
+      "_type": "Value"
+    },
+    "next.reward": {
+      "dtype": "float32",
+      "_type": "Value"
+    },
+    "next.done": {
+      "dtype": "bool",
+      "_type": "Value"
+    },
+    "index": {
+      "dtype": "int64",
+      "_type": "Value"
+    }
+  },
+  "homepage": "",
+  "license": ""
+}