added Gibbs sampling and species identifiers
added Gibbs sampling and species identifiers

Conjugate priors and Gibbs sampling are now used to update
hyper-parameters eta (hypZero) and she of exp prior on rates (hypR).
Additionally species identifiers for ts and te are now defined at the
beginning and only once.

dsilvestro/PyRate,dsilvestro/PyRate,dsilvestro/PyRate,dsilvestro/PyRate