Build a bifurcating tree when we get more than one child

[matsen]

Rather than adding the children one by one as we do now, we add all of them without sequences, or a dummy sequences, and then randomly resolve the polytomy. Once we've done that, we re-traverse the tree, mutating each child based on the parent, and then add the tips to updated_live_leaves. This process represents a "dark zone cycle" of reproduction, and we will also be able to use this for models with a non-trivial fitness.