Removed hard-coded (Dutch) values from capacity credit gquery.

As of https://github.com/quintel/etsource/commit/3bab0d2696a9b37ad4a0904714afb1d22e73a178, we capped the capacity credit of wind turbines to the Dutch value at 378 MW of installed wind capacity (i.e. 0.328) for installed wind capacities lower than 378 MW. These numbers come from the [thesis](http://refman.et-model.com/publications/1847) by Edgar Wilton. This cap is there to avoid that the capacity credit can increase to infinity for small installed wind capacities. Now that we are adding country dependent capacity credit curves, the hard coded values need to be replaced by dynamic numbers and we decided to cap the capacity credit to the present value for each country.