Michael Cohen spotting an error in this tutorial: we were looping over the cells vector after having created the cell population object, hence we were looping over nothing! Fixed this.

git-svn-id: https://chaste.cs.ox.ac.uk/svn/chaste/trunk@20154 b5f69653-2bf5-0310-8201-b0dafb8e1405
oops

git-svn-id: https://chaste.cs.ox.ac.uk/svn/chaste/trunk@20155 b5f69653-2bf5-0310-8201-b0dafb8e1405