Adding jupyter notebook illustrating the cameras transformation (#116)

* Adding jupyter notebook illustrating demonstrating how to extract camera parameters in the scene and how these camera parameters relate to the given views with proper testing.

test/test_relative_camera.py

+import os
+
+import pytest
+
+import habitat
+from habitat.datasets.pointnav.pointnav_dataset import PointNavDatasetV1
+
+try:
+    import torch  # noqa # pylint: disable=unused-import
+
+    has_torch = True
+except ImportError:
+    has_torch = False
+
+
+@pytest.mark.skipif(not has_torch, reason="Test needs torch")
+def test_demo_notebook():
+    config = habitat.get_config("configs/tasks/pointnav_rgbd.yaml")
+    config.defrost()
+    config.DATASET.SPLIT = "val"
+
+    if not PointNavDatasetV1.check_config_paths_exist(config.DATASET):
+        pytest.skip("Please download the habitat test scenes")
+    else:
+        pytest.main(
+            [
+                "--nbval-lax",
+                "notebooks/relative_camera_views_transform_and_warping_demo.ipynb",
+            ]
+        )