add isComplex flag to location-iterator to handle multiple units per instance

src/apps/canvas/structure-view.ts

src/mol-geo/representation/structure/complex-visual.ts

src/mol-geo/representation/structure/units-visual.ts

...esentation/structure/visual/carbohydrate-link-cylinder.ts

...presentation/structure/visual/carbohydrate-symbol-mesh.ts

...ntation/structure/visual/cross-link-restraint-cylinder.ts

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

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

src/mol-geo/util/location-iterator.ts