`CalcJobImporter`: consider replacing the `remote_data` input with a `Transport` instance

[sphuber]

By doing so, the transport throttling and bundling functionality of the engine will be used, allowing users to submit many importing jobs without overloading the server. The transport can be configured to change to the root dir of the remote data, such that the CalcJobImporter.parse_remote_data can simply use the Transport interface to retrieve the files of interest.