code: function distance(v) {
           var dx = this._x - v.x,
               dy = this._y - v.y,
               dz = this._z - (v.z || 0);
           return Math.sqrt(dx * dx + dy * dy + dz * dz);
         }
docs: return the distance between this vector and the passed one
  @name distance
  @memberOf me.ObservableVector3d
  @function
  @param {me.Vector2d|me.Vector3d|me.ObservableVector2d|me.ObservableVector3d} v
  @return {Number}