Added unary minus operator to SVector3

git-svn-id: https://geuz.org/svn/gmsh/trunk@19598 59c8cabf-10d6-4a28-b43f-3936b85956c4
Fixed computation and optimization of NCJ quality measure

git-svn-id: https://geuz.org/svn/gmsh/trunk@19599 59c8cabf-10d6-4a28-b43f-3936b85956c4