Add 'Instance' as a 'Data Type' to 'Sample Index'

Since an instance can exists of multple instances it would be nice to have 'Instance' as a 'Data Type' for the 'Sample Index' node.

I think this would be performance wise be better then 'Separate Geometry' where a 'Index' is checked with a 'Equal' node.

(Same for 'Evaluate at Index')