#2016 - fix two bugs that tripped STL assertions on Windows.

git-svn-id: https://chaste.cs.ox.ac.uk/svn/chaste/trunk@19941 b5f69653-2bf5-0310-8201-b0dafb8e1405
Fix r19941, and catch a couple more cases of the iterator decrement bug.

git-svn-id: https://chaste.cs.ox.ac.uk/svn/chaste/trunk@19942 b5f69653-2bf5-0310-8201-b0dafb8e1405
#2433 merge `ParallelCellsGenerator` with trunk

git-svn-id: https://chaste.cs.ox.ac.uk/svn/chaste/trunk@19943 b5f69653-2bf5-0310-8201-b0dafb8e1405