BDSKY 1.4.6 modified

This is a modified version of BDSKY v1.4.6, with diversified extant sampling and OU smoothing prior for the skyline fossilized birth-death model. This version works with BEAST v2.6.x. For BEAST v2.7.x or above, please use BDSKY v1.5.0 or later, which also has the methods implemented.