Make square centre-padding a model transform

This MR fixes issues #23 (closed) and #39 (closed), by:

• Removing centre-cropping from all databases raw-data loaders
• Creating a SquareCenterPad transform that operates on tensors
• Adding that transform to all relevant models (Pasa, Alexnet, Densenet)

conda/meta.yaml

@@ -43,6 +43,7 @@ requirements:
   run:
     - python >=3.10
     - {{ pin_compatible('click') }}
+    - {{ pin_compatible('grad_cam') }}
     - {{ pin_compatible('matplotlib') }}
     - {{ pin_compatible('numpy') }}
     - {{ pin_compatible('pillow') }}