Add panel metadata to AnnData export

[jwindhager]

... as variable annotations, as suggested by @ndamond

[jwindhager]

Also, concatenate multiple images, similar to the CSV export?

[ndamond]

Also, the image metadata (currently exported as images.csv) could be exported to AnnData "observations".

[jwindhager]

Unfortunately, anndata currently does not support pandas' nullable data types (see scverse/anndata#628), which are required for panel/image metadata. We'll have to wait for this to be resolved, before panel/image metadata can be included in the anndata export.

[ivirshup]

scverse/anndata#504 may be a better issue to track here

[jwindhager]

While nullable ints/bools are now supported, pandas StringArray support (for nullable string columns) is still missing: scverse/anndata#679

[ivirshup]

@jwindhager how strong is your need for strings instead of categoricals? E.g. will you have repeated values in these columns?

[jwindhager]

Thanks for checking in @ivirshup! Unfortunately, these cannot be expected to be categoricals. I'm currently implementing a workaround based on astype(str) (i.e., convert dtype("string") columns to dtype("O")) and leave it up to anndata to deal with it (I assume you're internally converting to categoricals - not the most efficient way I assume, but currently a good enough workaround I guess).

[ivirshup]

Hopefully that will work.

Alright, let me know if it doesn't. I'm still waiting for a better solution from pandas, but I think we could make this work given a use case.