made structure part of element and link loci

src/mol-model/shape/shape.ts

src/mol-model/structure/query/selection.ts

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

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

src/mol-repr/shape/index.ts

src/mol-repr/structure/units-visual.ts

src/mol-repr/structure/visual/carbohydrate-link-cylinder.ts

src/mol-repr/structure/visual/carbohydrate-symbol-mesh.ts

...ol-repr/structure/visual/cross-link-restraint-cylinder.ts

src/mol-repr/structure/visual/inter-unit-link-cylinder.ts

src/mol-repr/structure/visual/intra-unit-link-cylinder.ts

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

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

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