STYLE: Replace ITK_OVERRIDE with override

This commit replaces occurences of the ITK_OVERRIDE macro with override. This one-liner was used to perform the update:

git grep -l "ITK_OVERRIDE"  | fgrep -v CMakeLists.txt |fgrep -v .cmake | xargs sed -i '' -e "s/ITK_OVERRIDE/override/g"

It was adapted from https://github.com/InsightSoftwareConsortium/ITK/blob/master/Utilities/ITKv5Preparation/ReplaceITK_OVERRIDEMacroNames.sh