In that case you need two GridFunctions, which can be
bound to the two cells.

After consulting Steffen

It makes the code generation workflow more complicated without
contributing anything substantial

Instationary stuff not fixed so far.

Useful, if you are only interested in the jacobian_apply versions
anyway.

Still not beautiful...

The way we currently iterate over children of mxied elements
was plain wrong for nested mixed elements. The tests that
revealed this will soon follow.

The introduction of FunctionView turned out to be a major problem
with more complicated forms. The original idea was to preserver the
structure of the finite element in a way, that loops over components
of a mixed element are realized by actual loops (treating them with
free indices and such). However, this causes quite some nightmares and
was never implemented as generically as needed (I even doubt that is
possible).

However, there is another option, which is to unroll any such loops
on a symbolic level. While this may sound like a bad idea at first
there is some really positive aspects about it:
* ListTensor and ComponentTensor nodes collapse completely (and would
  otherwise have a big nightmare potential)
* Symbolic zeroes do not generate code - important in hyperbolic problems
  where the system matrices are quite sparse or for axiparallel grids,
  where geometric quantities have many zeroes.
* The compiler would unroll these small loops anyway.
* TSFC (and I guess also FFC) do it the same way.

Implementing this required me to redo the form splitting algorithm.
I rethought it and integrated it into the main ufl->loopy visitor.

Computes tensor index from single index.

Removes micro_to_macro_index from accum_insn and introduces a class BlockstructuredAccumulationSpace to specialize AccumulationSpace for blockstructured FEM