message: error handling when _atom_site.ihm_model_id is unavailable
diff: @@ -270,6 +270,10 @@ async function readIHM(ctx: RuntimeContext, format: mmCIF_Format, formatData: Fo
  const { ihm_model_list } = format.data;
  const entities: Entities = { data: format.data.entity, getEntityIndex: Column.createIndexer(format.data.entity.id) };
  + if (!format.data.atom_site.ihm_model_id.isDefined) {
  + throw new Error('expected _atom_site.ihm_model_id to be defined')
  + }
  +
  // TODO: will IHM require sorting or will we trust it?
  const atom_sites = splitTable(format.data.atom_site, format.data.atom_site.ihm_model_id);
  const sphere_sites = splitTable(format.data.ihm_sphere_obj_site, format.data.ihm_sphere_obj_site.model_id);
  