Calculate the Clade Concordance statistic of Nixon & Carpenter (1996)

[ms609]

Requires scores to be calculated on polytomies.

See Kearney 2002, Syst Biol, for a potential use wrt rogue taxa.

@article{Nixon1996,
  title = {On consensus, collapsibility, and clade concordance},
  author = {Nixon, Kevin C. and Carpenter, James M.},
  year = {1996},
  journal = {Cladistics},
  volume = {12},
  number = {4},
  pages = {305--321},
  doi = {10.1111/j.1096-0031.1996.tb00017.x}
}

@article{Kearney2002,
  title = {Fragmentary taxa, missing data, and ambiguity: mistaken assumptions and conclusions},
  author = {Kearney, Maureen},
  year = {2002},
  journal = {Systematic Biology},
  volume = {51},
  number = {2},
  pages = {369--381},
  doi = {10.1080/10635150252899824}
}