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...
> Also: using-declarations are encouraged to make units (and prefixes) readable.

I don't like the `using` directives because the sometimes lead to naming collisions which is exactly the issue namespaces are supposed to fix. (also the other places which use `Opm::unit` in this files did not use `using` either.) But I'm not dogmatic about this, if you _really_ want to have it change I'll update the commit...