Improve documentation highlighting where equations (3)-(5) in van Leeuwen et al (2009) are implemented in the Chaste code.

git-svn-id: https://chaste.cs.ox.ac.uk/svn/chaste/trunk@22466 b5f69653-2bf5-0310-8201-b0dafb8e1405
git-svn-id: https://chaste.cs.ox.ac.uk/svn/chaste/trunk@22472 b5f69653-2bf5-0310-8201-b0dafb8e1405
For #2558, add a `SetSolver` method to `AbstractCardiacCellInterface`.

git-svn-id: https://chaste.cs.ox.ac.uk/svn/chaste/trunk@22476 b5f69653-2bf5-0310-8201-b0dafb8e1405
For #2558, allow debugging barriers without recompiling (almost) everything.

git-svn-id: https://chaste.cs.ox.ac.uk/svn/chaste/trunk@22490 b5f69653-2bf5-0310-8201-b0dafb8e1405