Updating models to allow normal imports

This updates the python for all the models to allow normal importing in python3. There should not be any impact on the running of the model; everything will function as it did before, but the imports follow standard python rules instead of the slightly weird standards they did before.

In addition, there's a helper script that's been added to automatically update a model to make it importable, and there's a piece of the pre-commit hook that's been added to test if the models are all import-compatible.

make_model_importable.sh

@@ -12,7 +12,7 @@ perl -pi -e "s/^from function_library/from .function_library/g" ${1}/*.py
 perl -pi -e "s/^import propagators/from . import propagators/g" ${1}/*.py
 
 echo "Testing..."
-python3 -c "exec(\"import ${1}\".replace(\"/\",\".\"))"
+python3 -c "__import__(\"${1}\".replace(\"/\",\".\"))"
 if [ $? -ne 0 ]
 then
   echo "This doesnt seem to have worked. Please check by hand."