Cleanup `direct_sum.sigma_curry` / `direct_sum.is_internal.collected_basis`

The seems to be some duplication going on here : `direct_sum.is_internal.collected_basis` should maybe be a special case of `direct_sum.sigma_curry` (or the linear version). Moreover, there seems to be some inconsistencies between the vector space API and the finite dimensional Hilbert space API, and only some of these constructions exist for the latter.