#2327 Made subclass of `CellBasedPdeHandler` to hack in multiple Neuman Dirichlet boundaries

git-svn-id: https://chaste.cs.ox.ac.uk/svn/chaste/trunk@17930 b5f69653-2bf5-0310-8201-b0dafb8e1405
#2272 enable use of `CellwiseSourcePde` with a `MultipleCaBasedCellPopulation` on lattice simulation

git-svn-id: https://chaste.cs.ox.ac.uk/svn/chaste/trunk@17934 b5f69653-2bf5-0310-8201-b0dafb8e1405
#2312 Make !GetReadContainingElementOfBoundaryElement virtual so that it can be overridden by subclasses

git-svn-id: https://chaste.cs.ox.ac.uk/svn/chaste/trunk@17937 b5f69653-2bf5-0310-8201-b0dafb8e1405
Fix r17925 (#2328) for non-VTK build

git-svn-id: https://chaste.cs.ox.ac.uk/svn/chaste/trunk@17938 b5f69653-2bf5-0310-8201-b0dafb8e1405
git-svn-id: https://chaste.cs.ox.ac.uk/svn/chaste/trunk@17939 b5f69653-2bf5-0310-8201-b0dafb8e1405