Ensure events are sorted in time order

[JonathanDHarris]

Some code will require that event lists are in time order.

We should update the docs to reflect this
http://gamma-astro-data-formats.readthedocs.org/en/latest/events/index.html#required-column-names

and add to the check_times method
https://github.com/gammapy/gammapy/blob/master/gammapy/data/event_list.py#L721

I'll probably pick this issue up soon if no one else wants it

[cdeil]

I've moved this discussion to the format spec:
open-gamma-ray-astro/gamma-astro-data-formats#107
and once that's resolved we can adapt Gammapy.

[cdeil]

@adonath @registerrier - Let's try to resolve this old issue, please comment.

I think it won't be much work, but if we leave it unaddressed it probably means that some users get completely incorrect results without a warning. For sure we'll add a few lines of documentation, and concerning code changes it's either nothing, or a call to events.table.sort("TIME") or GTI table sort in one or a few places.

Do you know if there are any places in Gammapy where we rely on EVENTS or GTI being sorted by time?
Is gammapy.time.exptest the only one?
I think in lightcurve and other code, we use boolean masking and it doesn't matter how the tables are ordered?

It's not clear if we should support non-chronological EVENTS tables. The discussion in open-gamma-ray-astro/gamma-astro-data-formats#107 hasn't advanced, and probably it will take a long time for CTA to decide. So let's not wait for that. As far as I know, ctools does produce EVENTS that are non-sorted in ctobssim (at least it used to). I don't know about the Fermi tools.

If we don't rely on order: add docs saying so.

If we do rely on order in some places: we could sort at the start of those functions / methods, or raise an error. We could also sort on EVENTS (and GTI) read, although that isn't a good solution IMO.

Thoughts?

[cdeil]

I've added a note that we shouldn't rely on TIME being sorted in EVENTS here: 063a322

Otherwise I reviewed the code, there's no place where we rely on TIME sorting in EVENTS.

Closing this issue now.

Improving code and documentation concerning Time in Gammapy is covered by this issue:
#284 (comment)