basic support for .3dg files

src/mol-io/reader/3dg/parser.ts

src/mol-io/reader/_spec/3dg.spec.ts

src/mol-model-formats/structure/3dg.ts

src/mol-plugin/state/actions/data-format.ts

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

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