fullMatrix: added getDataPtr const and non const

git-svn-id: https://geuz.org/svn/gmsh/trunk@18511 59c8cabf-10d6-4a28-b43f-3936b85956c4
git-svn-id: https://geuz.org/svn/gmsh/trunk@18512 59c8cabf-10d6-4a28-b43f-3936b85956c4