handle polymer ends in visuals properly

...-model/structure/model/properties/utils/atomic-derived.ts

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

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

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

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