message: don't split small single atom polymer chains
diff: @@ -705,7 +705,8 @@ namespace Structure {
  const elements = SortedArray.ofBounds(start as ElementIndex, chains.offsets[c + 1] as ElementIndex);
  - if (singleAtomResidues) {
  + // check for polymer to exclude CA/P-only models
  + if (singleAtomResidues && !isPolymerChain(model, c)) {
  partitionAtomicUnitByAtom(model, elements, builder, multiChain, operator);
  } else if (elements.length > 200000 || isWaterChain(model, c)) {
  // split up very large chains e.g. lipid bilayers, micelles or water with explicit H
  @@ -733,6 +734,11 @@ namespace Structure {
  return model.entities.data.type.value(e) === 'water';
  }
  + function isPolymerChain(model: Model, chainIndex: ChainIndex) {
  + const e = model.atomicHierarchy.index.getEntityFromChain(chainIndex);
  + return model.entities.data.type.value(e) === 'polymer';
  + }
  +
  function partitionAtomicUnitByAtom(model: Model, indices: SortedArray, builder: StructureBuilder, multiChain: boolean, operator: SymmetryOperator) {
  const { x, y, z } = model.atomicConformation;
  const position = { x, y, z, indices };
  