Make discharge data from caravan available through the observations routine (as well as part of the forcing !)

The caravan dataset contains both data we consider "forcing" (Temperature, precipitation, potential evap, etc.) as well as discharge data we consider "observations". Currently we make this data accesable through 'forcing'.

For discharge data it would be great if we can load caravan discharge data similar to how we load GRPC data (and this prepares us for adding eStreams at a later stage). It is no problem if "under the hood" this would just call the forcing part and only returns the discharge part of the caravan forcing.