#2351 Fix Louie's deadlocking issue (caused by processes doing Send--then--Recv when messages are bigger than local buffers).

git-svn-id: https://chaste.cs.ox.ac.uk/svn/chaste/trunk@18400 b5f69653-2bf5-0310-8201-b0dafb8e1405
#2261 push the association of cells and location indices down from `AbstractCellPopulation` to its children to deal with the different cases when we do or do not have contiguous node / element indices. Also a small fix in `BoxCollection`. 

git-svn-id: https://chaste.cs.ox.ac.uk/svn/chaste/trunk@18401 b5f69653-2bf5-0310-8201-b0dafb8e1405