New faster version of ltt (& ltt95) for ultrametric trees

Some phytools users reported problems with the phytools function ltt95 (for plotting a 95% high probability LTT from a posterior sample of trees). This is likely due to the use of ltt internally, which is very slow. The reason it is slow is because it does not assume that the tree is ultrametric (& is probably unnecessarily slow even so, but that's a problem for another day). If we first check if the tree is ultrametric we can call branching.times internally which is fast, and everything else is sped along considerably.

I have now done that. The updated code is here & it is also part of a new phytools build (here) which can be downloaded & installed from source. When the trees in the posterior sample are ultrametric, the speed-up that results is really extraordinary (about 10× for a tree with 100 taxa).