[CreateInputsModel] Make `Qobs` parameter optional

The Qobs parameter is useless when the network in only composed of hydrological model nodes.

Qobs is used to fill the matrix Qupstream parameter of airGR::CreateInputsModel items but the column of this matrix is replace by upstream simulated flows during calibration and simulation.

The only exception are:

• when direct flows are injected in the network (case model = NA).
• when using option useUpstreamQsim = FALSE in Calibration

So my proposition is that:

• Qobs should be optional and in that case, Qupstream should be filled with zeros for airGR::CreateInputsModel calls.
• Qobs is mandatory only in case of nodes with model = NA and column names should at least match with these nodes.
• In case of using option useUpstreamQsim = FALSE, check in Calibration that all nodes are present in the columns.