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...
> I am sorry, but I do not understand what you think should be the expected
> from the functions?

in my opinion their names try imply that meaningless conversions are 
prevented, i.e.

convert::to(kg, Pa);

will either yield in a compiler error or at an exception. In reality it works 
just fine. Also it is very easy to "misuse" these functions even if the result 
is correct (as you have observed yourself).

> They exist precisely so that you do not need to
> remember "what does it mean when I multiply with (or divide with) this
> factor", but can read the intent straight out of the line. ```
> convert::to(rate, cubic(meter)/day)
> ```
> is a lot clearer than the alternative.

I think ratePerDay = ratePerSecond * secondsPerDay is not more unclear than 
dailyRate = convert:::to(ratePerSecond, 1/secondsPerDay) and has a lot less 
potential to do it wrong...
 
> The functions cannot do any meaningful error checking, because there are no
> conversions that are disallowed. 

true. that's why they should die ;)

> But they are not supposed to be used for
> converting (non-SI)->(other non-SI), and that could perhaps be clearer.

agreed, that's a different matter. Being hell-bend on keeping this functions 
the best alternative would probable be a convert(value, fromUnit, toUnit) 
function...