`instance_key` as categorical in PointsModel

[giovp]

Unrelated to the above but related to instance_key. I think that the column specified by instance_key should never be categorical, so we should remove this line:

spatialdata/spatialdata/_core/models.py

Line 464 in 4fb3433

if not is_categorical_dtype(data[instance_key]):

Originally posted by @LucaMarconato in #158 (comment)

[giovp]

so in this case, instance_key refers to points, not shapes. I agree it's debatable on whether we want to have it categorical. I think it might make sense in e.g. few regions but many points. I think few can be order of thousands of regions (and millions of points). wdyt? @LucaMarconato

[LucaMarconato]

Ah you are right! I had confused points and circles (have been working with napari today and they use the same name). It should be categorical I agree.