List all streams when no stream IDs are provided

[cbrnr]

Currently, mnelab.io.xdf.read_raw_xdf() requires stream_ids, a list of streams IDs to be imported. I think it would be convenient if it could spit out a list of all stream IDs if stream_ids=None (and then exit). Or maybe even better (because we want type stability of the return value), there should be a separate function that does the equivalent of pyxdf.resolve_streams(xxx), but perhaps with a more concise output.