Make cofunctionals terminal, and test

[dham]

When taking derivatives with respect to a Cofunction, ufl ends up creating ExprList objects containing cofunctions. Equality (==) then fails on the ExprList because the Cofunction is not identified as a terminal. This fixes that and puts in a test.

Also adds unicode methods for a couple of missing things that were hit along the way. The whole unicode conversion is untested and I'm not proposing to fix that here.