Implement no caching option

[mjp0]

At the moment Datagram caches everything it downloads into a local storage. Most of the time this is what the user wants, especially when using dg as a library, but this behavior leads to duplicate data when the use case is to export data from remote datagrams as files.

Each stream has already capability of cleaning up the cache, it just needs to be hooked up as an option.

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Deliverables

•  create nocache option for export
• implement cleaning to streams