Add validate_order function.

[danielballan]

This work was already started in #97 but in keeping with our new process of opening issues first to nail down the design, I'd like to move discussion here for the moment.

Scope should include:

• All foreign keys are preceded by the documents they reference. That means:

  • RunStart before the EventDescriptors that reference them
  • EventDescriptors for the Events that reference them
  • Datum before the Events hat reference them
  • Resources before the Datum that reference them

  Note that this does not mean that a RunStart must be first. For example, Resource is allowed to precede a RunStart.

• If a RunStop doc is present, it is the last doc. This is important because we sometimes use it as a signal to clean up resources (like Serializer.close()).

• Event[Page]s within each stream are in time order.

• Event[Page]s across streams are in time order up to the time resolution of a Page. That is, if we denote event_page['time'][0] as a_i ("a initial") and event_page['time'][-1] as a_f ("a final") for a given EventPage a, if b follows a and then b_f >= a_i. In English, each EventPage's highest time must be greater than or equal to the preceding EventPages' lowest times.

In terms of implementation, it would be good to avoid caching the entire run. It is possible to enforce these constraints without retaining every document in memory at once.