Working on structure data model

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

src/mol-model/structure/structure.ts

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

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

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

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

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