Makes invalid clusters return logPost of -NaN instead of NaN

-NaN makes the cluster guaranteed invalid. NaN has the opposite effect. I don't think we ever throw InvalidClusters inside the MH algorithm, but, if we do, they will be invalid before this fix.
Fixes bounds checking error in Montgomery's ageIter lower_bound search
Makes Bergeron filterSet error non-fatal
Version bump to 9.3.4
Updates yaml-cpp submodule