#2300 Refactored setting of pressure and flux boundary conditions to better support heterogeneous BCs

git-svn-id: https://chaste.cs.ox.ac.uk/svn/chaste/trunk@20927 b5f69653-2bf5-0310-8201-b0dafb8e1405


git-svn-id: https://chaste.cs.ox.ac.uk/svn/chaste/trunk@20928 b5f69653-2bf5-0310-8201-b0dafb8e1405