OBS_FLAG/CONF_STATUS split, confidential-slice no_results fix
- Breaking: eurostat_query_dataset's observations[].status carries only OBS_FLAG now; confidentiality moves to a new confStatus field, and the staged dataframe table gains matching conf_status/conf_status_label columns (#35)
- Fixed: a dataset slice where every cell is confidential no longer wrongly raises no_results (#36)
- OBS_FLAG/CONF_STATUS dictionaries and measure column names consolidated into a shared eurostat-codelists module; both dataframe tool descriptions updated to match