Add PyCml support for the `rem` operator, meaning LR94 can be converted.

git-svn-id: https://chaste.cs.ox.ac.uk/svn/chaste/trunk@21560 b5f69653-2bf5-0310-8201-b0dafb8e1405
Some pycml code tidying.

git-svn-id: https://chaste.cs.ox.ac.uk/svn/chaste/trunk@21561 b5f69653-2bf5-0310-8201-b0dafb8e1405