add a function


git-svn-id: https://geuz.org/svn/gmsh/trunk@17639 59c8cabf-10d6-4a28-b43f-3936b85956c4
git-svn-id: https://geuz.org/svn/gmsh/trunk@17640 59c8cabf-10d6-4a28-b43f-3936b85956c4