Tancik/scene contraction (#70)

Contract the scene to a sphere of radius 2. This is the same contraction used in MipNeRF 360.
Add mipnerf 360 graph and config. It currently doesn't contain the other advancements made in the paper (sample proposal network, regularizers)

docs/reference/fields/modules/index.rst

@@ -11,4 +11,5 @@ TODO: High level description of field modules and how they connect together.
    embeddings
    field_heads
    field_losses
-   mlp
+   mlp
+   spatial_distortions

docs/reference/fields/modules/spatial_distortions.rst

@@ -0,0 +1,8 @@
+.. _spatial_distortions:
+
+Spatial Distortions
+=====================
+
+.. automodule:: pyrad.fields.modules.spatial_distortions
+   :members:
+   :show-inheritance: