Return summed value of all descending spns
Examples
tree <- randTree(10)
getNdsPD(tree, ids = tree["all"]) # return PD of all ids
#> n1 n2 n3 n4 n5 n6 n7 n8
#> 11.107549 10.043449 8.850029 7.977889 6.953430 2.301880 2.916683 1.121731
#> n9 t1 t2 t3 t4 t5 t6 t7
#> 1.525615 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000
#> t8 t9 t10
#> 0.000000 0.000000 0.000000