Correctly handle intermediate station without hydrological model
Presently, only upstream nodes can be direct injections.
The modelling of an intermediary basin without any model defined (direct injection) is not clear.
I propose that injecting a direct flow on intermediary node is handled. The result would be that upstream simulated flows would not be routed to downstream.
With this way of doing, simulating run-of-river dam such as Pannecière Lake would be easier.