struc_conn as a custom property, mmcif export

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

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

...-model/structure/model/formats/mmcif/bonds/struct_conn.ts

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

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

...mol-model/structure/structure/unit/links/inter-compute.ts