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

@@ -25,3 +25,8 @@ tests:
     # for pytorch quirks on linux
     - if [ "$TAG" == "docker" ]; then export OMP_NUM_THREADS=1; fi # Set the number of threads used to 1
     - !reference [.snippets, test-prepare]
+
+conda-package:
+  before_script:
+    # for opencv-python dependence
+    - if [ "$TAG" == "docker" ]; then apt-get update && apt-get install -y libgl1-mesa-glx > /dev/null; fi