wip, basic gro format support

src/mol-io/reader/_spec/gro.spec.ts

src/mol-io/reader/cif/data-model.ts

src/mol-io/reader/gro/parser.ts

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

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

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

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

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