Fixed to understand rwty.trees objects

These three functions have had minor changes to make sure they know
what to do when they are fed an rwty.trees object.  They should now be
able to be called with rwty.trees or multiPhylo.
Improved comments and readability of compare.n

revised and expanded comments
Fixed bug with lapply causing tip label to be lost

replaced lapply with a loop, everything seems to work now
added test line for load.trees