WIP: Add network SNR threshold to BAYESTAR likelihood

BAYESTAR's likelihood now includes the network SNR threshold for detection. The default value is zero. For nonzero values, the likelihood broadens as the network SNR approaches its threshold value.

The network SNR threshold is set using the optional --net-snr-threshold command line argument, which is understood by the bayestar-localize-coincs and bayestar-localize-lvalert scripts.

Note, output of Bayes factors for signal-versus-noise (BNS) and coherence-versus-incoherence (BCI) are not working yet with this patch.