Forcing recipes: use fixed variables from different ensembles/scenarios

When generating forcing, sometimes a variable which is "fixed" is required. For example the orography used by the model.

As this variable is the same across all ensembles (or even different scenarios), usually this data is not stored more than once. This will cause users to find very few valid climate models & ensembles that they can use to generate their forcing, while the data is available somewhere.