admixture_edge: Create an admixture edge from a child to two parents.
In admixturegraph: Admixture Graph Manipulation and Fitting

Syntactic suggar for constructing edges in an admixture graph.

1	admixture_edge(child, parent1, parent2, prop = NA)

`child`	The name of the child node.
`parent1`	The name of the parent node.
`parent2`	The name of the parent node.
`prop`	Admixture proportions from `parent1` to `child`. If this parameter is not provided, you must explicitly specify the admixture proportion parameters in the `agraph` function call.