#1979 Updated description of .dat files

git-svn-id: https://chaste.cs.ox.ac.uk/svn/chaste/trunk@21772 b5f69653-2bf5-0310-8201-b0dafb8e1405
#2467 Forgot to use a distributed mesh. Changed to distributed and modified the tutorial to pass in a parallel case by using an `ElementIterator`.

git-svn-id: https://chaste.cs.ox.ac.uk/svn/chaste/trunk@21773 b5f69653-2bf5-0310-8201-b0dafb8e1405