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...
yep. I suppose that we also agree that for Runge-Kutta methods this value is not the average but that the time discretization needs to supply more values for the substeps which are definitively instantaneous...