wip, spheres renderable in visuals

src/mol-repr/representation.ts

src/mol-repr/structure/representation.ts

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

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

src/mol-repr/structure/units-visual.ts

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

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

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

src/mol-repr/visual.ts