Refactored Selection

src/mol-math/geometry/symmetry-operator.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/set.ts

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

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

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

src/perf-tests/structure.ts