added StructureFromTrajectory, improved multi-model structures

src/apps/viewer/extensions/cellpack/model.ts

src/mol-model/structure/query/queries/filters.ts

src/mol-model/structure/query/queries/generators.ts

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

src/mol-model/structure/query/utils/structure-set.ts

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

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

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

...mol-model/structure/structure/unit/links/inter-compute.ts

src/mol-model/structure/structure/util/subset-builder.ts

...l-model/structure/structure/util/unique-subset-builder.ts

src/mol-plugin/index.ts

src/mol-plugin/state/actions/structure.ts

src/mol-plugin/state/transforms/helpers.ts

src/mol-plugin/state/transforms/model.ts