use derived moleculeType and traceElementIndex when possible

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

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

src/mol-model/structure/util.ts

src/mol-repr/structure/visual/nucleotide-block-mesh.ts

src/mol-repr/structure/visual/util/nucleotide.ts

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

...l-repr/structure/visual/util/polymer/backbone-iterator.ts

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