#2183 tweak population writers to account for all possible combinations of template parameters.

git-svn-id: https://chaste.cs.ox.ac.uk/svn/chaste/trunk@18561 b5f69653-2bf5-0310-8201-b0dafb8e1405
git-svn-id: https://chaste.cs.ox.ac.uk/svn/chaste/trunk@18562 b5f69653-2bf5-0310-8201-b0dafb8e1405
#2183 add pure virtual method to cell population hierarchy to accept cell and population writers.

git-svn-id: https://chaste.cs.ox.ac.uk/svn/chaste/trunk@18564 b5f69653-2bf5-0310-8201-b0dafb8e1405