Allow negative ancestral outgroup height thresholds
Allow negative ancestral outgroup height thresholds

This lets us be a bit more conservative than the default
all-nodes-proceed-at-same rate assumption. We can still use ancestral
outgroups without needlessly bottlenecking ourselves when (inevitably)
one problem takes a little longer for whatever reason.

benedictpaten/cactus,benedictpaten/cactus,benedictpaten/cactus,benedictpaten/cactus,benedictpaten/cactus