use serendipity tetrahedron and haxahedron to compute high order interior vertices (instead of "unusual"... and now inexistant face-saturated incomplete element)

clean up high order routines by refactoring high-order tet/hex/prism/pyram generation code



git-svn-id: https://geuz.org/svn/gmsh/trunk@16632 59c8cabf-10d6-4a28-b43f-3936b85956c4