Addition of plugin DuplicateBoundary that allow one to modelize inter granular cracks with cohesive zones. 
Addition of the line command ComputeBestSeeds that allow one to optimize the placement of seeds in a microstructure to remove small edges from the Voronoï representation.
Small additions to cross3D and direction3D to compute the smoothness fo fields.

git-svn-id: https://geuz.org/svn/gmsh/trunk@19238 59c8cabf-10d6-4a28-b43f-3936b85956c4