Leaf stability indices

[ms609]

Implement Estabrook's leaf stability index, after

@article{Goloboff2015,
  title = {Identifying unstable taxa: efficient implementation of triplet-based measures of stability, and comparison with {{Phyutility}} and {{RogueNaRok}}},
  author = {Goloboff, Pablo A. and Szumik, Claudia A.},
  year = {2015},
  journal = {Molecular Phylogenetics and Evolution},
  volume = {88},
  pages = {93--104},
  publisher = {{Elsevier Inc.}},
  doi = {10.1016/j.ympev.2015.04.003}
}