Rename Column typealias to AnyColumnReference

[Jolanrensen]

Since all type aliases with a * are called AnySomething, it felt odd to have ColumnReference<*> be called Column. This insinuates that it is a Column object instance, which is not the case. AnyColumnReference (or a variant of that) seems more in line with the other aliases.

[koperagen]

I'm not sure if it was intentional or indeed an inconsistency
For example, groupBy operation syntax in documentation also uses column, columns https://kotlin.github.io/dataframe/groupby.html
What do you think?

[Jolanrensen]

So does update, but those are still of type ColumnReference<C>. It's odd that groupBy has the same type of argument, just unbound (ColumnReference<*>), but it's suddenly called Column. So, I'd suggest making those ColumnReference<*> or AnyColumnReference for clarity and consistency :)