join_sdata_spatialelement_table() fails with non-unique AnnData obs #494

When the AnnData object has obs that are not unique the join will fail because code like this table[joined_indices, :] require unique indices.

I think we should

• modify the function _get_joined_table_indices() to return numerical indices
• in general modify the loc in dataframes and the use of obs indices in anndata to use iloc and numerical indices instead

To test:

1. make a table with some rows and obs.index not unique
2. compute a left join