#2370 Simplify EM solvers so that they use contraction model specified by problem definition.

git-svn-id: https://chaste.cs.ox.ac.uk/svn/chaste/trunk@19027 b5f69653-2bf5-0310-8201-b0dafb8e1405
#2391 Resolve cell_based FPE issues.

git-svn-id: https://chaste.cs.ox.ac.uk/svn/chaste/trunk@19029 b5f69653-2bf5-0310-8201-b0dafb8e1405