Add select operation

[jmh530]

I was taking a look at R's data wrangling cheatsheet for use with dplyr. One function that does not seem to have a comparable option in magpie is select (unless I am mistaken), which is a lot like filter, but for columns. This would make a useful enhancement (as well as some of the other functionality there that is not yet in magpie).

[1] https://rstudio.com/wp-content/uploads/2015/02/data-wrangling-cheatsheet.pdf