REFAC: reduce forecast path loading

Target

There are many calls in PostProcessing retrieving forecast_path from data store. This leads to a messy tracking plot. Therefore, it would be better to load forecast_path once in PostProcessing.__init__ and use it as class attribute.

ToDos

• add self.forecast_path = self.data_store.get("forecast_path") in PostProcessing.__init__
• find all self.data_store.get("forecast_path") and replace by self.forecast_path