Bug fix in ltt()

One of my Phylogenetics students, Craig Sadowski, just found a bug in the latest version of my function ltt() which computes and creates a lineage-through-time plot; as well as calculating and hypothesis testing the γ statistic of Pybus & Harvey (2000). Basically, when I moved tol (a tolerance value for rounding) out of the function call, I neglected to assign it a value. Oops. Fixed in v0.5 (code here).