Making Cell Based PDE Methods Virtual to allow subclasses to override methods 

git-svn-id: https://chaste.cs.ox.ac.uk/svn/chaste/trunk@21649 b5f69653-2bf5-0310-8201-b0dafb8e1405
#2534 adding comments and making CA behaviour the same.

git-svn-id: https://chaste.cs.ox.ac.uk/svn/chaste/trunk@21654 b5f69653-2bf5-0310-8201-b0dafb8e1405
#2221 Deleting temp files

git-svn-id: https://chaste.cs.ox.ac.uk/svn/chaste/trunk@21655 b5f69653-2bf5-0310-8201-b0dafb8e1405