#2300 Adds an abstract acinar unit to allow easy setting up of different acinar models

git-svn-id: https://chaste.cs.ox.ac.uk/svn/chaste/trunk@22706 b5f69653-2bf5-0310-8201-b0dafb8e1405
#2300 Forgot to remove some old commented code

git-svn-id: https://chaste.cs.ox.ac.uk/svn/chaste/trunk@22707 b5f69653-2bf5-0310-8201-b0dafb8e1405
#2569 A new tutorial showing how to do equivalent mono and bidomain simulations.

git-svn-id: https://chaste.cs.ox.ac.uk/svn/chaste/trunk@22708 b5f69653-2bf5-0310-8201-b0dafb8e1405