added ihm extension to mmcif schema

data/mmcif-field-names.csv

data/mmcif-usage-counts.txt

src/apps/schema-generator/generate-typescript.ts

src/apps/schema-generator/schema-from-mmcif-dic.ts

src/apps/schema-generator/util/cif-dic.ts

src/apps/schema-generator/util/generate.ts

src/mol-io/reader/cif.ts

src/mol-io/reader/cif/schema/dic.ts

src/mol-io/reader/cif/schema/mmcif.ts