representation refactoring

src/mol-repr/index.ts

src/mol-repr/structure/index.ts

src/mol-repr/structure/representation/backbone.ts

src/mol-repr/structure/representation/ball-and-stick.ts

src/mol-repr/structure/representation/carbohydrate.ts

src/mol-repr/structure/representation/cartoon.ts

src/mol-repr/structure/representation/distance-restraint.ts

src/mol-repr/structure/representation/molecular-surface.ts

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

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