Add a minimum threshold for chisq

As noted in #51, spiir currently produces too many low reduced chisq zerolags.

We can offset this with a few thresholds:

1. Minimum chisq threshold (~0.3) for an IFO to contribute to cmbchisq (and only enabled ifos)
2. At least 1 IFO should be significant for gracedb upload: ~0.4 < chisq < ~3, snr >= ~4.0
   Note it's currently at least 2 IFOs, but that shouldn't be necessary. far, and possibly checks on cmbchisq, should separately make sure we're not getting false positives. It's still something we should verify though.
3. Low significance triggers should not skip other gracedb upload checks

These changes are reasonably small, but could be made separately. The actual values of the thresholds require tuning/testing. See spiir-production-Oct27-dev (and tdavies__remove_snr_constraint, it needs some changes split out though)

Once complete, we can try removing the 1.4 snr threshold: #43