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)

.gitlab-ci.yml

@@ -30,4 +30,5 @@ conda-package:
   before_script:
     # for opencv-python dependence
     - if [ "$TAG" == "docker" ]; then apt-get update && apt-get install -y libgl1-mesa-glx > /dev/null; fi
+    - mamba install -n base -y git
     - !reference [.snippets, conda-prepare]