#2300 Refactor resistance calculation -- fixing coverage

git-svn-id: https://chaste.cs.ox.ac.uk/svn/chaste/trunk@21323 b5f69653-2bf5-0310-8201-b0dafb8e1405
#2300 Iterative pressure solver (using direct flux solver).  Note that edges now have to be topologically sorted (parent branch should appear before both children in list of edges).

git-svn-id: https://chaste.cs.ox.ac.uk/svn/chaste/trunk@21325 b5f69653-2bf5-0310-8201-b0dafb8e1405
#2300 Check that iterative and PETSc solutions are comparable.

git-svn-id: https://chaste.cs.ox.ac.uk/svn/chaste/trunk@21327 b5f69653-2bf5-0310-8201-b0dafb8e1405