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...
> Unit conversion: the from() function should convert from a non-SI-base unit to SI

true. but that only applies if the unit to be converted is in the nummerator. in this case it is in the denominator, which makes everything confusing. (and thus I'd prefer to simply use "*" and "/" directly...)