Abstract download_file protocols across devices due to similarity

[davidverweij]

Just a note for now, but this is very similar to the dreem implementation (which is fine), so perhaps in a future PR we can abstract it out into a shared task rather than device specific? since the only difference between dreem/VTT at the moment is how data is downloaded (line 103).

This is food for thought on how we might abstract this further once we implement the DAGs

Originally posted by @jawrainey in #21 (comment)

[davidverweij]

This now also includes other protocols, such as the recently introduced uid_to_hash() in combination with all_hashes() methods.

[jawrainey]

Agreed -- also pulling out device agnostic logic such as __device_id_from_ucam in dreem.py since it determines a device in UCAM by a wear period and DeviceType (needs renaming too).

[jawrainey]

Worth noting now that while there is some overlap, BTF differs in that it returns a filename rather than bool. I argue that DRM and other pipelines should change to follow BTF for consistency.