Adds enumeration to generated code that gives (dccl.msg).id as DCCL_ID and (dccl.msg.max_bytes as DCCL_MAX_BYTES
Added (dccl.msg).unit_system option to provide a default unit system for all fields in the message. Also, required that the unit_system be explicitly set either using (dccl.msg).unit_system or in (dccl.field).units.system