The triplet distance is calculated as the sum of different outgroups among
every triplet of tips between the two trees. Normalisation is performed by dividing
the resulting number by the total number of triplets shared between the two trees.
The triplet distance is calculated only for shared tips between the two trees. Parallelizable.
References
Critchlow DE, Pearl DK, Qian C. (1996) The Triples Distance for rooted bifurcating phylogenetic trees.
Systematic Biology, 45, 323-34.