Tensor index notation library node

[JanKleine]

For reference, here is the code from my thesis. Most of the code is in the branch of this PR, but some smaller parts (e.g. the transformation prototype) may be in the tensor-index-notation-library-node-proto or tensor-index-notation-library-node branches of my fork.

This PR should not be merged as is. Some cleanup is likely coming later, as discussed with Phil.