OU calculations on non-ultrametric trees

[richfitz]

See email from Pennell (7/12/2013), http://www.zoology.ubc.ca/prog/diversitree/examples/ou-nonultrametric/ and

library(diversitree)
set.seed(2)
tree <- rtree(100)
x <- rTraitCont(tree)
lik.ou.v <- make.ou(tree, x, control=list(method="vcv"))
lik.ou.p  <- make.ou(tree, x,
                     control=list(method="pruning", backend="C"))