_splitBranch(parentBranch) {
    // if the branch is shaded with a node, replace the node in the new subset.
    let containedNode = null;
    if (parentBranch.childrenCount === 1) {
      containedNode = parentBranch.children.data;
      parentBranch.mass = 0;
      parentBranch.centerOfMass.x = 0;
      parentBranch.centerOfMass.y = 0;
    }
    parentBranch.childrenCount = 4;
    parentBranch.children.data = null;
    this._insertRegion(parentBranch, "NW");
    this._insertRegion(parentBranch, "NE");
    this._insertRegion(parentBranch, "SW");
    this._insertRegion(parentBranch, "SE");

    if (containedNode != null) {
      this._placeInTree(parentBranch, containedNode);
    }
  }
this function splits a branch into 4 sub branches. If the branch contained a node, we place it in the subbranch
after the split is complete.
@param {object} parentBranch
@private