str and repr for UFL forms is spewing millions of warnings.

@martinal updated UFL on Monday such that calling str or repr on a form spews forth a warning. We call str on forms at solving.py:137 and repr at timeforms.py:86 . This means that dolfin-adjoint currently spews pages of warnings.

I think at solving.py:137 we could use hash instead of str. I'm not so sure about timeforms.py:86