wip, strongly type model indexing

src/mol-data/int/segmentation.ts

src/mol-model/structure/model.ts

...odel/structure/model/formats/mmcif/secondary-structure.ts

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

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

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

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

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

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

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

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