Add odds ratio of association vs random coincidence; fixes #21

This adds an odds ratio between a joint candidate being a real coincidence versus being from a random association. Default values return mean, lower, and upper 90% credible bounds based on predicted rates for O4.

Default rates values from 10.1093/mnras/stz455 but should be later updated from further studies. GRB rate is identical to GCN rate. The noise rate for GWs comes from https://igwn.readthedocs.io/projects/gwcelery/en/latest/gwcelery.conf.html#gwcelery.conf.superevent_far_threshold

This function is meant to advise RRT about a joint candidate rather than used as a criteria for publishing a public alert, based off of arxiv.org/abs/2111.12814.