Add create function to finite_element and dof_map

I would like to add factory functions in UFC, in particular

  finite_element* finite_element::create() const;
  dof_map* dof_map::create() const;

Are there any objections to this? It would be easy to add and easy to
implement in FFC.

I need it to implement refinement of FunctionSpaces in DOLFIN where
fresh copies of ufc::finite_element and ufc::dof_map are needed given
a ufc::finite_element or ufc::dof_map.

An alternative is to hack create_sub_foo so that it returns a copy of
the element/dofmap itself if the component argument 'i' is >= the
number of sub elements/dofmaps.