Improve ergonomics by overloading getindex

An `OutputVar` is a named array with metadata attached to it. It would be convenient to be able to index into a `OutputVar` by doing something like `var[1, 2, 3]` or `var[time = 1:3]`. There is already support for this with `ClimaAnalysis.select`. which `getindex` can call. It is unclear to me if `var[1,2,3]` should be supported, since it is not obvious what is being indexed.