add tubular helices to Cartoon representation

CHANGELOG.md

src/mol-model-props/computed/helix-orientation.ts

...del-props/computed/helix-orientation/helix-orientation.ts

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

src/mol-repr/structure/visual/polymer-trace-mesh.ts

src/mol-repr/structure/visual/polymer-tube-mesh.ts

src/mol-repr/structure/visual/util/polymer/trace-iterator.ts