From what I can tell, this makes the code harder to read than if we just used tuples:
```
grib1_key = _Grib1ToCfKeyClass(table2_version, centre_number, param_number)
... <code omitted> ...
cf_data = _GribToCfDataClass(standard_name=cfdata.standard_name,
                             long_name=cfdata.long_name,
                             units=iris_units,
                             set_height=height)
... <code omitted> ...
_GRIB1_CF_TABLE[grib1_key] = cf_data

```
vs
```
... <code omitted> ...
_GRIB1_CF_TABLE[(table2_version, centre_number, param_number)] = 
    (cfdata.standard_name, cfdata.long_name, iris_units, height)
```