Obnam: unifying access to local and remote repositories, part 2

Goal Today's goal is to finish the work of having a unified way to access local and remote repositories. Plan Change the ClientRepository type to be an enum instead of struct . This will make it possible for obnam/src/bin/cmd/mod::open_repository to return values for local or remote repositories. The current approach of using a trait means only one concrete type can be returned. Change anything…