message: improve Structure.asParent
  handle parent coordinate system not identity
diff: @@ -384,10 +384,8 @@ class Structure {
  asParent(): Structure {
  if (this._proxy) return this._proxy;
  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);
  + const p = this.parent.coordinateSystem.isIdentity ? this.parent : Structure.transform(this.parent, this.parent.coordinateSystem.inverse);
  + const s = this.coordinateSystem.isIdentity ? p : Structure.transform(p, 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;
  