wip, strongly typed indexing

src/mol-geo/representation/structure/visual/util/polymer.ts

...-model/structure/export/categories/secondary-structure.ts

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

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

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

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

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

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

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

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

src/mol-model/structure/query/utils/builders.ts

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

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

src/mol-model/structure/structure/util/subset-builder.ts