Make Cols more flexible

[bkamins]

Possible useful additions:

• Cols(<:Type)
• Cols(<:Function)

that would match how the names function works.

@nalimilan - the only limitation is that currently AbstractIndex is not aware of data frame columns, and we handle Cols on AbstractIndex level. I will have to think how to handle this cleanly (as the simplest solution is to add special methods on AbstractDataFrame level, but it is not nice as then some of the methods for handling columns would be defined at index level, and some at data frame level).

[bkamins]

Adding Cols(<:Type) this would require a significant redesign of the package. We would have to pass around AbstractDataFrame or DataFrameRow in places were we currently pass around AbstactIndex (and there are many such places). Also with DataFrameRow is would be especially tricky to add.

I would constrain ourselves for now to Cols(<:Function) which should be easier.