Conditional inclusion in CIF writer fields

src/mol-io/writer/cif/encoder.ts

src/mol-io/writer/cif/encoder/binary.ts

src/mol-io/writer/cif/encoder/text.ts

src/mol-io/writer/cif/encoder/util.ts

src/mol-math/geometry/symmetry-operator.ts

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