GTI required or optional with EVENTS?

[cdeil]

Should GTI (Good time interval) extensions be required or optional? Are they required to be in the same file as the EVENTS extension?

@mimayer - What's the status in ctools? Are there any concrete plans to change something? http://gamma-astro-data-formats.readthedocs.org/en/latest/events/index.html#disclaimer-about-names-of-hdu-and-file-structure

Does someone have time to look at the EVENTS and GTI specs from OGIP? Do existing FTOOLs handling EVENTS work if no GTI is present (for operations where it's not needed)? This is probably something where we should just follow the existing practices, no?