Flow-thru demand mass balance error

[Enrique T]

MODSIM converges to a solution that has mass balance errors in the flow-thru returns. The attached network illustrates the issue. Flows shown at the FThru return nodes are the right amounts, but the flows in the links downstream of the return node differ from the correct solution.

[Bob Lounsbury]

Duplicate of #38.

[Bob Lounsbury]

@etriana fyi. The code has this warning for ExchangeDems:

Warning:
 currently exchange demands do not understand water that is going
 to satisfy a demand that is not 100% consumptive or a flow through.

So the solution is not correct even after the bug is fixed so that flows match in the return node and downstream link. You might want to add an enhancement issue to add the capability you're after if there is not another way to capture what you are trying to model.

[Bob Lounsbury]

@etriana That took a long time, but I think I may have found one of the real issues. The convergence check on flow thru returns was looking at the wrong numbers so it was converging prematurely. It is now iterating to what appears to be a valid solution. Will hopefully be closing this flow thru bug soon.