mmCIF export, mandatory category prefix for model custom props

src/mol-model/structure/export/categories/atom_site.ts

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

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

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

...mol-model/structure/model/properties/custom/descriptor.ts

src/perf-tests/mol-script.ts

src/servers/model/properties.ts

src/servers/model/properties/rcsb/symmetry.ts

src/servers/model/properties/structure-quality-report.ts