basic microheterogeneity support

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

src/mol-model/sequence/sequence.ts

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

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

src/mol-plugin/ui/sequence/polymer.ts

src/mol-plugin/ui/sequence/residue.tsx

src/mol-theme/label.ts