add support for Beta and Gamma peptides

src/mol-model-formats/structure/common/entity.ts

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

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

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

...-model/structure/model/properties/utils/atomic-derived.ts

...l-model/structure/model/properties/utils/atomic-ranges.ts

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

src/mol-model/structure/structure/accessible-surface-area.ts

src/mol-model/structure/util.ts

src/mol-plugin/behavior/dynamic/labels.ts

src/mol-repr/structure/visual/util/polymer/trace-iterator.ts

src/mol-theme/color/molecule-type.ts

src/mol-theme/color/secondary-structure.ts