#1992 - some tidying up of Megan's code, which should be slightly more efficient too.

git-svn-id: https://chaste.cs.ox.ac.uk/svn/chaste/trunk@17898 b5f69653-2bf5-0310-8201-b0dafb8e1405
#1660 Added (commented out) failing test for post processing data being written to the VTK file.

git-svn-id: https://chaste.cs.ox.ac.uk/svn/chaste/trunk@17899 b5f69653-2bf5-0310-8201-b0dafb8e1405