Add "std::string *::ufl() const" for all relevant classes

   class form
   {
     // Name describing coefficient function i.
      // (F.ex. "f" for the typical right hand side,
      // "mu" and "lambda" for linear elasticity
      // and "Re" for Navier-Stokes).
      std::string coefficient_name(unsigned int i) const = 0;

      // Name of physical unit, if any, for coefficient function i.
      std::string coefficient_unit(unsigned int i) const = 0;

      // Name of integrals (could also be put in integral classes).
      std::string cell_integral_name(unsigned int i) const = 0;
      std::string exterior_facet_integral_name(unsigned int i) const = 0;
      std::string interior_facet_integral_name(unsigned int i) const = 0;

      // Name of underlying library or code generation tool.
      std::string library_name() const = 0;

      // Name of author.
      std::string author() const = 0;

      // Implementation in UFL, if applicable.
      std::string ufl() const = 0;

    };

AL: I suggest naming the ufl function something like ufl_repr so it's clear it's a string that can be used to instantiate a UFL expression in Python. It would also be good to make it non-pure virtual and return "" by default. Then form compilers or hand coders that don't depend on UFL don't need to implement this function. UFC should not depend on UFL.