Add 'PBS()'

[atongsa]

PBS (population branch statistics) [^1] is derived from Fst, which can detect positive selection signals in admixted populations .

the formula is

T = - log(1 – FST)

PBS = (T_TH + T_TD − T_HD) / 2

[^1]: Yi, X. et al. Sequencing of 50 Human Exomes Reveals Adaptation to High Altitude. Science 329, 75–78 (2010).