homoplasy_mutations: Get probabilities of mutation along all branches
In guerreror/pepo: Estimate Hemiplasy and Homoplasy on a Phylogeny

Given a species tree, calculate the probability of mutation on each of the branches of the gene tree.

1 2	homoplasy_mutations(this, desc_a, desc_b, sib, anc, mu_rate, coal = T, mode)

`this`	Numeric. Branch length of focal internal branch.
`desc_a`	Lengths of branches descending from focal branch.
`desc_b`	Lengths of branches descending from focal branch.
`sib`	Length of sibling branch (leading to tip node C).
`anc`	Length of ancestral branch (the ABC ancestor)
`mu_rate`	Numeric of length 2. Population-wide mutation rate (forward, reverse)
`coal`	logical. Did coalescence in the BC ancestor happen?
`mode`	String. One of the three models implemented: "standard" (default), "minimal", or "strict".