Introducing parallel face in ReferenceSpace -- Doc review

git-svn-id: https://geuz.org/svn/gmsh/trunk@16515 59c8cabf-10d6-4a28-b43f-3936b85956c4
Oops: HexRefSpace in CMakeList but without code

git-svn-id: https://geuz.org/svn/gmsh/trunk@16516 59c8cabf-10d6-4a28-b43f-3936b85956c4