model type improvements, readonly maps

src/mol-model/sequence/sequence.ts

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

src/mol-model/structure/model/formats/mmcif/sequence.ts

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

...ol-model/structure/model/properties/chemical-component.ts

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