Parameter estimation on Burst-BBH triggers associated with low-significance CBC triggers

This partially solves #808. The idea is to search for the associated CBC trigger with the minimum FAR when preferred event is cWB-BBH. This MR is not expected to change any PE behavior when preferred event is from a CBC pipeline. Pinging gwcelery, bilby, and rapidpe developers. cc: @roberto.depietri @colm.talbot @caitlin.rose