CreateInputsModel: downgrade error on "`Qobs` only for nodes Direct Injection and Diversion" to warning

#99 (closed) restricts the use of the Qobs in CreateInputsModel since the data is not use in other case than "Direct Injection", "Diversion" or "Reservoir" nodes.

This error is blocking for old codes especially first versions of airGRiwrm where the examples populated Qobs with observed flows at all nodes.

It's preferable to turn it out into a warning saying that the columns not corresponding to the 3 kind of nodes handling observed flows will be ignored in the process.