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).

that's a very reasonable assumption and the function names try to imply this quite hard, but if you actually look at their implementation, you'll find this:

```c++
namespace Opm {
        namespace convert {
            inline double from(const double q, const double unit)
            {
                return q * unit;
            }
           inline double to(const double q, const double unit)
            {
                return q / unit;
            }
        }
```

i.e., the names of the function imply that they can detect error cases which they actually can't. In my opinion this means that it is better to kill them if nobody wants to extend them to do the expected things...