Support use of reservoir outflow data

[hboisgon]

Feature type

Adding new functionality

Improvement Description

The current SimpleReservoir model does not easily allow to control reservoir outflows even when data is available probably because we also need to tune parameters such as the min and max target volumes or the shape of the s curve used.
In case reservoir outflow data is available directly, it would be nice to have a more simple reservoir concept where the outflow is not computed from demand or rating curves but directly uses timeseries provided by the user (still checking if enough water is available in the reservoir volume).

Implementation Description

In cases where the rating curve or Wflow model approach is not precise enough to correctly predict outflows, it could be useful to use user-defined outflow timeseries directly in Wflow.

To allow for this, a new reservoir outflow variable should be added to the Wflow model and if there is enough water available in the reservoir, the reservoir outflow is then equal to the user-defined outflow rather than calculated using a rating curve. Similar to RTC-Tools, it would be useful if a check or switch could be implemented so that a user could easily either use the normal rating curve concept or the user-defined outflows if provided in operational use (ie easy to switch from one concept to the other in the toml file).

As for the inflow and abstractions issue #464, it could be useful if the format of the user-defined reservoir outflows timeseries could be in CSV or NetCDF-CF_TIMESERIES format rather than NetDCF_CF_GRID to avoid extra processing.

Additional Context

Timely and accurate forecast is crucial to predict and respond to flood or drought risk. For reservoir operators or basins that contains reservoirs, adapting reservoir operations in time might help reduce the impact of drought or floods. During our work with Sunwater, we noticed that in forecast mode, it is important that reservoir models, here Wflow, can reflect accurately reservoir operations such as:

Modification of the release outflow and/or rating curves.
Account for varying abstractions from the reservoirs (e.g. for irrigation).
If not considered properly, there is a risk that the model starts differing from the observations, reducing the accuracy of the forecast.

This issue is part of these improvements.