Detect if a form is self-adjoint (symmetric)

[Martin Sandve Alnæs]

Implement algorithm to determine whether or not a bilinear form is self-adjoint (symmetric). Form compilers could exploit this information in code generation and could inform an assembler.

Pseudo-steps for such an algorithm could be:

      def canonical_form(expression):
          return expression transformed to a truly canonical form, i.e. a representation that is completely unique
      self_adjoint = True
      for each integrand:
          a = canonical_form(integrand)
          b = canonical_form(replace(integrand, {v:u}))
          if not a == b:
              self_adjoint = False
              break

Note that a truly canonical form does not exist for the entire algebra supported by ufl, so some theoretical false negatives may be unavoidable.