git-svn-id: https://chaste.cs.ox.ac.uk/svn/chaste/trunk@18864 b5f69653-2bf5-0310-8201-b0dafb8e1405
#2042 change `NodeBasedCellPopulation::mNodePairs` to a vector for speed and make it possible to switch off the calculation of  `mNodeNeighbours` for speed, if desired

git-svn-id: https://chaste.cs.ox.ac.uk/svn/chaste/trunk@18867 b5f69653-2bf5-0310-8201-b0dafb8e1405