Support offline mode for NSRR records

[cbrnr]

Once NSRR records are downloaded locally, there should be no need having to log in to sleepdata.org. This is currently the case, and this can be problematic if the website is down.

[cbrnr]

I know that the readers already support offline=True, but I meant some automatic settings like e.g. offline="auto" (which could be the default?), which would only download in case the requested files are not available locally. But I'm not sure if this is really necessary.