Load incomplete data, collaboration meeting demos

[JelleAalbers]

This includes the strax changes and notebooks used for the demonstration I gave at Coimbra. Specifically:

• A new context option allow_incomplete. If set, and the storage backend supports it, it will allow users to load data that has not yet been completely written. When set, all saving is blocked and generates a warning instead, just like for the fuzzy and time range options.
• Support for allow_incomplete in the files backend. To enable this, the files backend now writes metadata to disk every chunk, which may cause a tiny slowdown. If a forked saver is used to save the data (e.g. raw_records during DAQ reading) allow_incomplete will not work yet.
• Some extra validation of context options: you now get a warning if you specify an unknown option.
• Protection against a common error: trying to set new options by passing a positional-arg dict to get_xxx will now result in an error. The correct way of setting new config options is passing config=dict(...), see the tutorial in the docs.
• Silence numerical warnings in the high-level XENON plugins around the statements where NaNs are known occurrences. These come from design choices like using NaN for cs1 when the event has no valid s1-s2 pair.
• Updated notebooks used in the demo.