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...
You can either convert::from() something->SI, or convert::to() SI->something. It does not matter at all if a unit is in the numerator or denominator, that is reflected by writing the unit properly (with a / in it if appropriate).