message: support coordinateSystem in structure.asParent
diff: @@ -383,7 +383,15 @@ class Structure {
  */
  asParent(): Structure {
  if (this._proxy) return this._proxy;
  - this._proxy = this.parent ? new Structure(this.parent.units, this.parent.unitMap, this.parent.unitIndexMap, this.parent.state, { child: this, target: this.parent }) : this;
  + if (this.parent) {
  + if (!this.parent.coordinateSystem.isIdentity) {
  + throw new Error('parent coordinate system must be identity');
  + }
  + const s = this.coordinateSystem.isIdentity ? this.parent : Structure.transform(this.parent, this.coordinateSystem.matrix);
  + this._proxy = new Structure(s.units, s.unitMap, s.unitIndexMap, { ...s.state, dynamicBonds: this.dynamicBonds }, { child: this, target: this.parent });
  + } else {
  + this._proxy = this;
  + }
  return this._proxy;
  }
  