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)

doc/img/Makefile

+# SPDX-FileCopyrightText: Copyright © 2024 Idiap Research Institute <contact@idiap.ch>
+#
+# SPDX-License-Identifier: GPL-3.0-or-later
+
+MAINFILES=data-model.dot
+LITE_PNGFILES=$(MAINFILES:%.dot=%-lite.png)
+DARK_PNGFILES=$(MAINFILES:%.dot=%-dark.png)
+
+.PHONY: FORCE_MAKE clean
+
+all: $(LITE_PNGFILES) $(DARK_PNGFILES)
+
+%-lite.png: %.dot FORCE_MAKE
+	dot -Tpng -Gbgcolor="white" -Nfontcolor="black" -Ncolor="black" -Efontcolor="black" -Ecolor="black" -o $@ $<
+
+%-dark.png: %.dot FORCE_MAKE
+	dot -Tpng -Gbgcolor="black" -Nfontcolor="white" -Ncolor="white" -Efontcolor="white" -Ecolor="white" -o $@ $<
+
+clean:
+	@rm -vf $(LITE_PNGFILES) $(DARK_PNGFILES) $(shell find . -name "*~")