Handle simple in-line reservoir inside the model

Until now, reservoirs are handle through direct injections and only upstream nodes can be direct injections.

As a result, catchment with online dams like the Pannecière Lake are currently complicated to integrate in the GRiwrm structure.

Here's below the situation of the Pannecière Lake:

graph LR
P(PANNECIERE)
CHAUM_07 -->|0km| P
P --> |153km| GURGY_02
style P fill:#99f

And it's representation in airGRiwrm:

graph LR
CHAUM_07 --> |153km| GURGY_02
PANNEC_P --> |153km| GURGY_02
PANNEC_R --> |153km| GURGY_02
style PANNEC_P fill:#fcc
style PANNEC_R fill:#fcc

Because of the unicity of the network, we have to first manually simulate the upstream basin, copy it's value multiplied by -1 into the intake of the lake (PANNEC_P) in order to annihilate the upstream influence and only routing the release of the lake (PANNEC_R) downstream.

A solution could be to keep the first scheme above and use a "direct injection" node which will simply returning the flow provided in Qobs for the node corresponding to the dam.

graph LR
CHAUM_07 --> |0km| PANNEC_R
PANNEC_R --> |153km| GURGY_02
style PANNEC_R fill:#fcc

With this simple solution, we can remove simply the preliminary simulation of the upstream part of the dam and the complex step of substracting all of upstream contribution.