added nucleotide blocks visual

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

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

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

src/mol-math/linear-algebra/3d/mat4.ts

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

src/mol-model/structure/util.ts

src/mol-view/stage.ts