_freezeNodes() {
    const nodes = this.body.nodes;
    for (const id in nodes) {
      if (Object.prototype.hasOwnProperty.call(nodes, id)) {
        if (nodes[id].x && nodes[id].y) {
          const fixed = nodes[id].options.fixed;
          this.freezeCache[id] = { x: fixed.x, y: fixed.y };
          fixed.x = true;
          fixed.y = true;
        }
      }
    }
  }
When initializing and stabilizing, we can freeze nodes with a predefined position.
This greatly speeds up stabilization because only the supportnodes for the smoothCurves have to settle.
@private