remove 3dg in favor of g3d (#93)

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/formats/trajectory.ts

src/mol-plugin-state/objects.ts

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