Implement DataAPI.refarray/refpool

[quinnj]

No description provided.

[bkamins]

@nalimilan recently also mentioned implementing it. But if I remember correctly there was some issue with a correct handling of missings (do not remember the details now unfortunately)

[nalimilan]

Yeah, unfortunately that’s trickier because you need to handle missing values (ref 0). So we need to return an object which can be indexed with 0. I considered using CategoricalPool for that, but that requires making it aware of whether its parent array allows for missing values (via a type parameter).

[nalimilan]

See #303.