sorting imports on save; cleanup imports sorting; add isort check (#207)

tests/fields/modules/test_encodings.py

@@ -3,6 +3,7 @@ Encoding Tests
 """
 import pytest
 import torch
+
 from nerfactory.fields.modules import encoding
 
 

tests/fields/modules/test_field_outputs.py

@@ -1,9 +1,9 @@
 """
 Field output tests
 """
+import pytest
 import torch
 from torch import nn
-import pytest
 
 from nerfactory.fields.modules.field_heads import (
     DensityFieldHead,