It is badly implemented as is and does not offer value over
writing symbolic functions.

We never really used or trusted it.

Do not annotate FiniteElements anymore, further split driver code

Conflicts:
	python/dune/perftool/pdelab/driver/taskbased.py

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.