Table region_key column is not categorical anymore after concatenate() #221

While working on the Lundeberg data I have a list of SpatialData objects for which the table has a region_key column with a categorical dtype. After concatenating the table I get a table which has dtype object for that column.

I am simply doing

sdata_patient1_visium_expression.table.obs['region'] = sdata_patient1_visium_expression.table.obs['region'].astype('category')

for the moment, but we'll have to fix this (I think the problem is concatenate() but I haven't investigated), add tests, and remove the line above from the Lundeberg notebook.