Generates the context group registry ETF from priv/context_groups.json.
Reads the scraped PS3.16 context group data, resolves include chains,
and writes the registry as a pre-compiled ETF binary that
Dicom.SR.ContextGroup.Registry loads at compile time.
Usage
mix dicom.gen_context_groups # uses priv/context_groups.json
mix dicom.gen_context_groups path/to/cg.json # custom path