Simulates nested phylogenetic trees (gene trees in species tree,
symbiont trees in host trees) using birth-death processes and transfers
between lineages. Simulations of gene trees within species trees are performed
using a three-tree model with species trees, locus trees, and gene trees.
The cophylogenetic birth-death process is used to simulate sets of host and
symbiont trees with extant associations between tips.
For more information about the three-tree model see:
Mallo et al. (2015)