DDM: cleaning cylinder case

git-svn-id: https://geuz.org/svn/gmsh/trunk@19985 59c8cabf-10d6-4a28-b43f-3936b85956c4
Adding corner DDM problem for getdp

git-svn-id: https://geuz.org/svn/gmsh/trunk@19986 59c8cabf-10d6-4a28-b43f-3936b85956c4
Cleaning DDM

git-svn-id: https://geuz.org/svn/gmsh/trunk@19987 59c8cabf-10d6-4a28-b43f-3936b85956c4
FunctionSpace is able to remove keys

git-svn-id: https://geuz.org/svn/gmsh/trunk@19988 59c8cabf-10d6-4a28-b43f-3936b85956c4