Provide spatially aggregated weatherforecast

In the current status of the campaign planing design workflow we want to visualize the precipitation forecast as an ordinary timeseries (one aggregated value per timestep for the entire ROI). Since the current version of the weatherprediction function returns a spatio-temporal datacube, we need to add the spatial aggregation (e.g. sum the raster values up).

This might be implemented by the Statistics module #3 (closed) .