wip, polymerElements, gapElements as unit props

...resentation/structure/visual/polymer-backbone-cylinder.ts

...epresentation/structure/visual/polymer-direction-wedge.ts

...o/representation/structure/visual/polymer-gap-cylinder.ts

...geo/representation/structure/visual/polymer-trace-mesh.ts

src/mol-geo/representation/structure/visual/util/polymer.ts

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

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