Support combined supply and drainage

[visr]

Over Outlets and Pumps in the text below I only mention Outlets for brevity. Related to Deltares/Ribasim-NL#221

For Ribasim-NL we want to model Outlets and enable flow over them in two cases:

• drainage of upstream Basin: set appropriate min_upstream_level and max_downstream_level, $D_u$ and $D_d$
• supply of downstream Basin: set appropriate min_upstream_level and max_downstream_level, $S_u$ and $S_d$

Now we want to model a single structure that can be used for both cases. We can use two parallel Outlet nodes as done in this test model. This has some issues:

• In the Dutch model many structures will need two nodes, not ideal for recognizability
• If we have simultaneously drainage and supply we'd get double flows. This can happen if upstream water levels are too high, and downstream too low, although in practice I suspect this won't be reached quickly because the supply and drainage cases would each work against getting here.

@ghendrickx proposed adding this scenario directly to Ribasim, for instance by having two rows in the static table with the same control_state. With this we can set flow for supply OR drainage to avoid double flows. If we were to allow different Q for the two cases we'd also need a way to select the right Q. So perhaps it would be wise to stick to a single Q.

This explains the situation in a h-h plot where upstream level (Hu) is on one axis and downstream level (Hd) on the other:

Another way to implement this is to use DiscreteControl with 4 conditions:

1. $h_u > D_u$
2. $h_u > S_u$
3. $h_d > D_d$
4. $h_d > S_d$

If we encode this using the logic table, such that FTFF means only condition 2 is true, we can fill in our plot:

This would allow us to avoid double flows. One difference with using min_upstream_level and max_downstream_level is that there are no reduction factors, which may increase simulation time if there is frequent switching between cases. Although we can combine the two approaches. If we have control_state S, D, and S+D, we can easily determine appropriate flow_rate for each, and set min_upstream_level and max_downstream_level as well to still get reduction factors at the edges of our cases. These should then only be set for S and D, not S+D because you don't want to reduce flows when moving from S+D to S or D.

I think the DiscreteControl option is preferable to double Outlets. But in discussing the issues users can have with double Outlets the idea of compound structures came up, so I created #2074 for that.