Morphology/brittle star 2 arms (#17)

Provide the features to read in Environment (Morphology, arena, ...) config files in the JSON format. The example JSON file contains the config for a brittle star with 2 arms

configs/example.json

@@ -0,0 +1,8 @@
+{
+  "morphology": {
+    "num_arms": 2,
+    "num_segments_per_arm": 4,
+    "use_p_control": true,
+    "use_torque_control": false
+  }
+}