Support ragged arrays in AnnData.obs

[alam-shahul]

I am trying to write a ragged array (an adjacency list) to the AnnData.obs field. However, when I try to reload this from the h5 file, it seems each of the inner lists is encoded as a string instead of as an array. However, it seems the HDF5 does support variable length arrays (see here); could anndata possibly support this?

[ivirshup]

We're working on this.

See draft PR here #647 and previous discussion #609.

[alam-shahul]

Thanks!