better support for polymers with a coarse backbone, improved MoleculeTypeAtomRoleIds

src/mol-model-formats/structure/mmcif/atomic.ts

src/mol-model/structure/model/properties/atomic/hierarchy.ts

...-model/structure/model/properties/utils/atomic-derived.ts

...ol-model/structure/model/properties/utils/atomic-index.ts

...l-model/structure/model/properties/utils/atomic-ranges.ts

src/mol-model/structure/model/types.ts

src/mol-model/structure/util.ts

src/mol-repr/structure/visual/polymer-trace-mesh.ts

src/mol-repr/structure/visual/util/polymer/trace-iterator.ts