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...
> This is a strange conversion: it should be cubic(meters)/day, shouldn't it?

nope: the unit of `rate()` m^3/s and I read `Opm::unit::convert::from()` as "*", so it multiplies "m^3/s" with "s/day" (because  Opm::unit::day is equivalent to 24*60*60) which will yield "m^3/day", so it should be correct. This is exactly the kind of confusion I was talking about in #651...