#2467 Added a new tutorial for conductivity modifiers, taken from TestBidomainTissue.

git-svn-id: https://chaste.cs.ox.ac.uk/svn/chaste/trunk@21763 b5f69653-2bf5-0310-8201-b0dafb8e1405
#2467 Tweaks for tutorial formatting.

git-svn-id: https://chaste.cs.ox.ac.uk/svn/chaste/trunk@21765 b5f69653-2bf5-0310-8201-b0dafb8e1405