On infeasible solution restart - groundwater return flows are not set to zero

[Bob Lounsbury]

If a model is allowed to restart with infeasible solution issues, groundwater return flows start from where they stopped rather than being reset to zero.

[Bob Lounsbury]

Closes #2. Demand previous flows not reset to zero on infeasible solution restart.

Several other fixes are in this commit too: * Rent pool contributor not limited to capacity minus contribution. * Storage flow step only links convergence issues due to return flows. * Reservoirs balanced at every time step rather than as specified in model. * Reservoir accrual links received less allocation because reservoir outflow was not included in calculation. * Owner links to flow thru nodes returns were forced into the storage step, returns are now available for natural flow allocation.

→ <<cset f1b40c4f05bd>>