Column.unique

[MarcoGorelli]

Any objections to adding Column.unique to get the unique values in a Column?
It could return another Column with just the unique values.

Why do we need this? From #131 (comment), if we're disallowing iterating over groups, then the workaround is to find the unique values, filter by them, and apply the user-defined function each time

[rgommers]

adding a unique method to columns seems perfectly reasonable to me. I'm not even sure it needs that detailed a rationale - unique is a pretty fundamental operation for dataframe libraries I'd think, and hence having that as a method on both dataframes and columns seems expected.

[kkraus14]

+1 on the function

We may want to document any ordering guarantees or lack thereof as well.

[jorisvandenbossche]

We may want to document any ordering guarantees or lack thereof as well.

Probably the lack of any ordering guarantee? So neither sorted (what numpy does) or order of appearance (what pandas does)?