code: function add(v) {
           return this._set(this._x + v.x, this._y + v.y, this._z + (v.z || 0));
         }
docs: Add the passed vector to this vector
  @name add
  @memberOf me.ObservableVector3d
  @function
  @param {me.Vector2d|me.Vector3d|me.ObservableVector2d|me.ObservableVector3d} v
  @return {me.ObservableVector3d} Reference to this object for method chaining