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 (so the result is suitable for use internally in the simulator. The to() function converts from SI (as used internally in the simulator to something else. The function call as written is read as: convert the number (rate here) from using day as unit, to SI. I think what you want is:
```
convert::to(rate, cubic(meter)/day)
```
This assumes that it is indeed correct to return from this function in that unit. I do not see an output statement, so I am sceptical, perhaps the conversion should rather be at the call site?

Using-declarations and directives: using them inside functions is appropriate, and encouraged. There will be no naming collisions from a `using Opm::unit' inside a function.