Instationary stuff not fixed so far.

We recently learned that usage of sub_elements in UFL is ambiguous...

Even if the basis scalar, it is worth to put a shape of (n, 1),
because that is how PDELab thinks about it. Not doing so can have
you end up with mismatches between `Dune::FieldVector<double, 1>` and `double`.

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.

I should work on a specific FiniteElement class or FEM for blockstructuring, s.t. I can remove the element parameter from the blockstructured implementation as well.

Computes tensor index from single index.

Moves application of transformation from micro to reference element from visitor to PDELab interface