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

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

Unrolling the dimension loops broke the diagonal_jacobian
transformation since it was looking for index_sums over certain
expressions.

It was the wrong level of asbtraction!