Fix rate conversion

Note that the conversion code now uses Opm::unit::convert::from() instead of simply multiplying with the conversion factor. I still think this obfuscates things more than it helps, but @bska will hopefully appreciate this...
Anyway, I now changed this line to

```
            value = sign_ * Opm::unit::convert::to(wellState.wellRates()[flatIdx_],
                                                   1 / Opm::unit::day);
```

I hope that it's okay now,,,