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? 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.

The functions cannot do any meaningful error checking, because there are no conversions that are disallowed. But they are not supposed to be used for converting (non-SI)->(other non-SI), and that could perhaps be clearer.