ENHANCEMENT: to_legacy_anndata support with region > 1

[gboscagli]

Hi all,

I managed to reproduce this notebook you also presented in the 2024 SpatialData paper, regarding Visium slide integration on a prostate from a unique patient. My main intention was to test SpaTrack, a spatially-aware trajectory inference tool, on multiple slides at once instead of on individual slides, since those multiple slides belong to the same tumoral focus and hence a unique trajectory is highly desirable instead of one per Visium dataset.

However, SpaTrack queries the AnnData object, so I tried to use to_legacy_anndata with the integrated coordinate_system, and encountered this (expected) error:

ValueError: The table needs to annotate exactly one element. Found 3.

I think it would be nice to have a full retrocompatibility also for this (IMHO) very interesting use case. What do you think about it?