Add a new handy cell property method.

git-svn-id: https://chaste.cs.ox.ac.uk/svn/chaste/trunk@22583 b5f69653-2bf5-0310-8201-b0dafb8e1405
Const some things that should be const

git-svn-id: https://chaste.cs.ox.ac.uk/svn/chaste/trunk@22585 b5f69653-2bf5-0310-8201-b0dafb8e1405