Adds an ipython notebook illustrating simple use cases (#106)

The habitat-api-demo.ipynb notebook illustrates how to specify config parameters, print semantic annotations of a scene. It also visualises the RGB, semantic and depth observations obtained by executing a random sequence of actions.

test/test_demo_notebook.py

+import os
+
+import pytest
+
+import habitat
+from habitat.datasets.pointnav.pointnav_dataset import PointNavDatasetV1
+
+
+def test_demo_notebook():
+    config = habitat.get_config("configs/tasks/pointnav_mp3d.yaml")
+    config.defrost()
+    config.DATASET.SPLIT = "val"
+
+    if not PointNavDatasetV1.check_config_paths_exist(config.DATASET):
+        pytest.skip(
+            "Please download the Matterport3D PointNav val dataset and Matterport3D val scenes"
+        )
+    else:
+        pytest.main(["--nbval-lax", "notebooks/habitat-api-demo.ipynb"])