#2517 Taken caching out of CardiacElectroMechanicsProblem and put it into AbstractConductivityModifier and corrected for monodomain/bidomain differences.

git-svn-id: https://chaste.cs.ox.ac.uk/svn/chaste/trunk@21498 b5f69653-2bf5-0310-8201-b0dafb8e1405
Record release 3.1 in trunk's changelog, for posterity.

git-svn-id: https://chaste.cs.ox.ac.uk/svn/chaste/trunk@21500 b5f69653-2bf5-0310-8201-b0dafb8e1405