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

It was the wrong level of asbtraction!

We avoid the bug by not applying the transformation and casting the
result of jacobianInverseTransposed to a FieldMatrix. This way we do
not exploit the diagonal structure of the Transformation. Should get
fixed in the near future.

Example: Before this commit the expression

\sum_i a_i (\nabla v)_i + \sum_j b_j (\nabla v)_j

was split into two accumulation terms:

1) a_k with corresponding test function (\nabla v)_k
2) b_l with corresponding test function (\nabla v)_l

Now we split into:

a_k + b_k with corresponding test function (\nabla v)_k

This is possible since we have linearity in the test function.

TODO: Jacobians are not yet working

But only when they are actually used!

As std::string::c_str() returns it.