Optimize json storage

[Bisaloo]

The angular app is getting quite slow. This is directly related to the size of forecasts_to_plot.json.

When inspecting forecasts_to_plot.json, one can notice that data is not stored in an optimal way and there is a lot of redundant data.

We should think about how to re-organize it.

The only problem is that this will require changes in the angular app as well.