Refactored AtomicKeys to AtomicIndex and added more types of lookups

src/mol-model-props/pdbe/structure-quality-report.ts

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

...-model/structure/model/formats/mmcif/bonds/struct_conn.ts

...ucture/model/formats/mmcif/pair-restraints/cross-links.ts

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

src/mol-model/structure/model/properties/sequence.ts

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

...mol-model/structure/model/properties/utils/atomic-keys.ts

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