Wind downscaling experiments

For the envisioned benchmark paper, downscaling of the wind field as inspired for instance by Höhelein et al., 2020 is targeted. However, some adjustments to the code are required to handle a vector quantity since the number of (effective) predictands gets increased from one target to two target variables.
The adaptions of the code will take place in this issue-branch. Note that this issue-branch will be based on #61 (closed) to integrate the new data loader and the benchmarking back to the develop-branch as well.