MetricBasis: better bounds


git-svn-id: https://geuz.org/svn/gmsh/trunk@18286 59c8cabf-10d6-4a28-b43f-3936b85956c4
git-svn-id: https://geuz.org/svn/gmsh/trunk@18287 59c8cabf-10d6-4a28-b43f-3936b85956c4