more specialized location-iterators

.../representation/structure/visual/nucleotide-block-mesh.ts

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

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

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

...ol-geo/representation/structure/visual/util/nucleotide.ts

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

...resentation/structure/visual/util/polymer/gap-iterator.ts

src/mol-geo/util/color-data.ts

src/mol-view/theme/color/unit-index.ts