More atom set refactoring

src/mol-model/structure/_spec/atom-set.1.spec.ts

src/mol-model/structure/_spec/atom-set.spec.ts

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

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

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

src/mol-model/structure/structure/atom/impl/builder.ts

src/mol-model/structure/structure/atom/impl/set.1.ts

src/mol-model/structure/structure/atom/impl/set.ts

src/mol-model/structure/structure/atom/set.1.ts

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

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

src/perf-tests/sets.ts

src/perf-tests/structure.ts