Fix #2418 by coping with units-converted state variables in the Jacobian.

git-svn-id: https://chaste.cs.ox.ac.uk/svn/chaste/trunk@19714 b5f69653-2bf5-0310-8201-b0dafb8e1405
#2410 beef up test a bit.

git-svn-id: https://chaste.cs.ox.ac.uk/svn/chaste/trunk@19716 b5f69653-2bf5-0310-8201-b0dafb8e1405