Fixed the capacity credit function for wind.

It turned out that by reducing the number of wind turbines considerably, you could achieve a capacity credit for wind of more than 100%. This has to do with the fact that the fit function we use is only valid for installed wind capacities of 379 MW and more. To fix this, I inserted an IF statement in `capacity_credit_wind.gql`. This IF statement makes sure that, if the user reduces the installed capacity for wind to below 379 MW, the capacity credit is fixed at the value that the fit function returns for 379 MW, i.e. `0.328`.

In addition, I fixed the `capacity_credit` attribute of the three `energy_power_wind` converters to match the value that the fit function returns for the start year.