#2323 Give `NodesOnlyMesh` sole control of its `BoxCollection` and hide it as much as possible. Give `NodesOnlyMesh` power to grow its underlying `BoxCollection` if the nodes get too close to the boundary. Also a few tidies in `NodesOnlyMesh` of commented code etc.

git-svn-id: https://chaste.cs.ox.ac.uk/svn/chaste/trunk@18220 b5f69653-2bf5-0310-8201-b0dafb8e1405
#2323 dox

git-svn-id: https://chaste.cs.ox.ac.uk/svn/chaste/trunk@18221 b5f69653-2bf5-0310-8201-b0dafb8e1405