Calculate spacetime FAR by default, temporal FAR as a backup if sky maps not available
Since Erik, Tri, and I are have converged on the space-time false alarm rate calculation and the RAVEN review has generally agreed that the space-time FAR can be interpreted as a FAR, @erik-katsavounidis and @tito-canton have suggested it would be best calculate the space-time FAR whenever possible. If either sky maps are not available we should default back to the temporal coincidence FAR.