Use NtC tube and separate representations for nucleic and protein

src/apps/rednatco/api-impl.ts

src/apps/rednatco/api.ts

src/apps/rednatco/density-map-controls.tsx

src/apps/rednatco/index.tsx

src/apps/rednatco/traverse.ts

src/apps/rednatco/viewer.ts

src/extensions/dnatco/behavior.ts