Structure data model tweaks

src/mol-data/util/equivalence-classes.ts

src/mol-geo/representation/structure/index.ts

src/mol-geo/representation/structure/point.ts

src/mol-geo/representation/structure/spacefill.ts

src/mol-geo/representation/structure/utils.ts

src/mol-geo/theme/structure/color/index.ts

src/mol-geo/theme/structure/size/index.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