Rename _filter to a public function name in ColSpec

[juliohm]

Currently we write _filter(spec, names) in the transforms. Maybe we should rename _filter to a name without underscore to signal that it is part of a "public" api of ColSpec.

[eliascarv]

OK.
We can't use filter because it's a base function. So we have to think of another name for this function.
What do you think of selnames?

[juliohm]

What about choose(spec, cols)?

[eliascarv]

I think 'choose' is a good name. We must chage 'colspec' to 'spec'?
snames = choose(spec, names)

[juliohm]

I think colspec is fine if it is already the term used in the current implementation and use cases of the function.

[eliascarv]

OK