Pin clang-format version for format check

This provides a consistent clang-format version. Also adds a script CI/check_format_local which takes the same argument as CI/check_format but uses the same container as the CI. This means that applying format through that script locally (should) guarantee that the formatting is consistent with the CI, since it uses exactly the same version.

Thoughts? @msmk @rlangenb

CI/check_format

@@ -14,7 +14,18 @@ if [ $# -ne 1 ]; then
     exit 1
 fi
 
+clang-format --version
+
 cd $1
 find . -iname '*.cpp' -or -iname '*.hpp' -or -iname '*.ipp' \
   | xargs clang-format -i -style=file
+
+
+if ! [ -z $CI ]; then
+  mkdir changed
+  for f in $(git diff --name-only); do
+    cp --parents $f changed
+  done
+fi
+
 exec git diff --exit-code --stat