Sample divergence function parameter c for a(t) = c / (1-t)^2 through Gibbs sampler
sample_c_two(shape0, rate0, tree_structure)a numeric value of the newly sampled c
shape of the Inverse-Gamma prior
rate of the Inverse-Gamma prior
a data.frame containing the divergence times and number of data points to the left and right branches of internal nodes on the tree