Blockstructured tests may fail, since the interpolation of the exact solution is not suitable

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.

This should probably be part of the loopy Mapper but I'm not sure if I could add it there.

This should probably be part of the loopy Mapper but I'm not sure if
I could add it there.