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

[ADedecker]

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.