Xenium conversion of indexes to str

[LucaMarconato]

index.equal also checks for the index values. They are str for obs columns from AnnData objects, while they are range indices for pandas dataframes. This PR converts the indices of the latter to str.

Note that the index is never used downstream (we use the instance_key column instead), so an alternative approach would have been to change the assert statements to have them compare the values of the columns, and not the indices.

[LucaMarconato]

Also this small commit was needed: dda3ffb