Reduce tree expand boilerplate code

[RAMitchell]

This PR reduces the below code, used in various updater algorithms into a single line. This reduces boilerplate code and increases safety. Before you could add child nodes without any split in the parent node.

p_tree->AddChilds(nid);
(*p_tree)[nid].SetSplit(e.best.SplitIndex(), e.best.split_value, e.best.DefaultLeft());
// mark right child as 0, to indicate fresh leaf
(*p_tree)[(*p_tree)[nid].LeftChild()].SetLeaf(0.0f, 0);
(*p_tree)[(*p_tree)[nid].RightChild()].SetLeaf(0.0f, 0);

New code:

p_tree->ExpandNode(nid, e.best.SplitIndex(), e.best.split_value, e.best.DefaultLeft());

[trivialfis]

Looks good to me.