Specify defaults via run metadata / run doc

[JelleAalbers]

This allows runs to specify default values for strax processing options via their metadata (run documents / database) using the strax_defaults field. This is very similar to the processor... field in XENON1T and will hopefully make correction handling a lot easier. Strax configruration priority is now as follows (high to low):

• Context options (set by user)
• Defaults specified in run metadata (i.e. run document/database)
• Defaults specified with plugin definition, using default, default_factory or default_per_run (in that order of priority).

The default_per_run option will soon be deprecated/removed , now that this new way of specifying run-dependent defaults is available. Currently default_per_run forces strax runs to have names that are castable to integers, which breaks proper functioning of superruns (as @petergaemers noted) and is a pain more generally.

The original motivation behind default_per_run was that I liked to enable processing independent of a runs DB. Since run metadata can for some time already be provided by json files (as we currently do on dali), that motivation no longer applies.

[JelleAalbers]

Hmm, this is less useful than I thought: we should only put options in the run doc that will never change (or keep some kind of history, and think carefully about how this interfaces with the context-based versioning in straxen). Otherwise it becomes difficult to go back and reproduce old results.

Nonetheless, (a) this could be useful for non-changing settings (b) it's a XENON1T-like backup option, useful for small TPCs etc, and (c) there are things in this PR we'll want anyway to support the upcoming deprecation of default_per_run.