support for recommended iso value volumes

src/apps/viewer/embedded.html

src/apps/viewer/index.ts

src/mol-model-formats/structure/common/property.ts

src/mol-model-formats/volume/ccp4.ts

src/mol-model-formats/volume/cube.ts

src/mol-model-formats/volume/density-server.ts

src/mol-model-formats/volume/dsn6.ts

src/mol-model-formats/volume/dx.ts

src/mol-model-formats/volume/property.ts

src/mol-model/volume/volume.ts

src/mol-plugin-state/actions/volume.ts

src/mol-plugin-state/formats/volume.ts

src/mol-plugin-state/transforms/volume.ts