mol-plugin: refactored selection transforms

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

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

src/mol-plugin/state/objects.ts

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

src/mol-plugin/util/structure-measurement.ts

src/mol-plugin/util/structure-query.ts

src/mol-script/script.ts