message: fix Structure.unitIndexMap creation when units are unsorted
diff: @@ -275,13 +275,15 @@ class Structure {
  for (let i = 0, _i = units.length; i < _i; i++) {
  const u = units[i];
  unitMap.set(u.id, u);
  - unitIndexMap.set(u.id, i);
  elementCount += u.elements.length;
  polymerResidueCount += u.polymerElements.length;
  if (u.id < lastId) isSorted = false;
  lastId = u.id;
  }
  if (!isSorted) sort(units, 0, units.length, cmpUnits, arraySwap);
  + for (let i = 0, _i = units.length; i < _i; i++) {
  + unitIndexMap.set(units[i].id, i);
  + }
  this._props.elementCount = elementCount;
  this._props.polymerResidueCount = polymerResidueCount;
  return { unitMap, unitIndexMap };
  