[ENHANCEMENT] decide `eachindex` future

[ogauthe]

Currently eachindex returns an error. This is associated with the length and the quantum dimension being different. We need to decide how do handle it once GradedUnitRange is rewritten.

Discussed at #5 (comment)

[ogauthe]

We settle on returning an error for eachindex and use a different function to return the fusiontrees. So I think this can be closed.