Small update to default arguments of reroot

I just updated the phytools function reroot so that the default position along the edge at which to re-root the tree is at the node.number node supplied by the user. Previously, the function would not run when position was not supplied.

E.g.:

library(phytools)
plotTree(tree,offset=0.8)
nodelabels()
tiplabels()

Now, let's perform operations on this original tree:

## reroot 0.1 along edge leading to node 22
t_22<-reroot(tree,node.number=22,position=0.1)
plotTree(t_22)

## reroot halfway along edge leading to tip "D"
node<-which(tree$tip.label=="D")
position<-0.5*tree$edge.length[which(tree$edge[,2]==node)]
t_D<-reroot(tree,node,position)
plotTree(t_D)

## reroot at node 16
t_16<-reroot(tree,16)
plotTree(t_16)

## reroot at tip "A"
plotTree(t_A<-reroot(tree,which(tree$tip.label=="A")))

Note that in this function the argument position is the distance from the rootward end of the edge leading to node node.number at which the tree should be re-rooted. Also, though it is hard to demonstrate here, the function now has an interactive=TRUE mode (described here).