Add Series.join

[adamklein]

It'd be nice to have a join function, such as s1.Join(s2) that creates a Frame with two columns from two series with int col keys. Saves a bit of typing here and there.

[tpetricek]

There are Series.zip functions which do the same thing as join - but on series. The naming is a bit unfortunate (should we use "zip" as in sequences or "join" as in frames)?

The zip functions on series are not as nice because (unless you have inner join), you'll end up with Series<K, OptionalValue<V1> * OptionalValue<V2>>.

But maybe we could add "join" functions that return the result as frame with two columns?

[adamklein]

Yeah, the join function could be as simple as unpacking the Series<K, V1 opt * V2 opt> into a Frame.