Fetch semantics for datasets

[dustmop]

cafs.Filestore and cafs.Fetcher both relate to how datasets act with regards to getting file data from the underlying store. The only difference between Fetch and the Filestore's Get is that Fetch takes a source parameter (meant to represent any vs local vs ...), but the source is currently being ignored. We should determine what specifically is different between Get and Fetch, probably that Fetch is aware of locations, but Get should only Get from specific sources, and cleanup the usage of both everywhere.