Add confirmation step for downloads

[trey-stafford]

Some kind of confirmation step is probably useful, esp. for large downloads. Currently, the primary entrypoint to interaction with iceflow takes dataset search parameters and automatically downloads matching granules if they are not already downloaded to the specified output dir.

Perhaps it would be good to expose a slightly lower-level API as well, that lets users more easily bypass the search & download phase, if they already have some data on disk that they know they want to interact with directly.