Use UFL element hashing instead of FInAT element hashing (#3807)

* Fixes #3732.

firedrake/output/vtk_output.py

@@ -453,13 +453,11 @@ def __init__(self, filename, project_output=False, comm=None, mode="w",
     def _prepare_output(self, function, max_elem):
         from firedrake import FunctionSpace, VectorFunctionSpace, \
             TensorFunctionSpace, Function
-        from tsfc.finatinterface import create_element as create_finat_element
 
         name = function.name()
         # Need to project/interpolate?
         # If space is not the max element, we must do so.
-        finat_elem = function.function_space().finat_element
-        if finat_elem == create_finat_element(max_elem):
+        if function.function_space().ufl_element() == max_elem:
             return OFunction(array=get_array(function),
                              name=name, function=function)
         #  OK, let's go and do it.