Add minimum chisq threshold for coincs

This MR adds a minimum chisq threshold for coincs, including uploads and xmls written to disk.

Previously, we require each upload to have at least 2 active detectors with chisq between 0 and 3, while also requiring all pairs of detectors to have a ratio of their chisq < 8
e.g. if one detector has a chisq of 1, then no other detector can have a chisq > 8.
If one detector has a chisq of 0.1, then no other detector can have a chisq > 0.8.

As I understand it, generally IFOs with very low chisq will also have very low snr, so it's probably not significantly contributing to the trigger. We wouldn't want to throw out a good 2 IFO trigger due to a third IFO with low signal.

At the same time, triggers with a single bright IFO are more likely to be glitches (though we're testing that case with other changes).

Requiring 0.3 min chisq on 2 detectors and only applying chisq ratio requirements on those detectors helps with those cases. The root of the issue is likely that we only coarsely exclude IFOs that are in a gap, but if just some samples of a trigger are in a gap, it could drag down the IFO's chisq. That can be addressed as a separate change.

That's my understanding of the change, though I believe @manoj.kovalam has more details. We've been using it for a while and rather than cherry-pick it into each test, I'm inclined to get it merged, and iterate from there as required.