wip, dsn6 support for volume server packing

src/mol-io/reader/ccp4/parser.ts

src/mol-io/reader/dsn6/parser.ts

src/servers/volume/pack.ts

src/servers/volume/pack/format.ts

src/servers/volume/pack/format/ccp4.ts

src/servers/volume/pack/format/dsn6.ts

src/servers/volume/pack/main.ts