wip, register ball-and-stick

package-lock.json

src/apps/canvas/structure-view.ts

src/mol-repr/structure/registry.ts

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

src/mol-repr/structure/visual/element-sphere.ts

src/mol-repr/structure/visual/intra-unit-link-cylinder.ts

src/mol-repr/structure/visual/util/element.ts