Computes the Jetz DR rate. Code modified from fisse
DR_statistic(x)an ape::phylo object
vector of rates for each tip
DR_statistic(ape::rcoal(10))
#> t2 t3 t1 t6 t9 t5 t10 t8
#> 1.313374 1.483246 1.483246 1.165268 4.598895 4.598895 2.616511 4.060064
#> t4 t7
#> 4.060064 2.373435