Implement constraints for pressure reducing valves

[tasseff]

We will model simplified versions of pressure reducing valves, compared to how EPANET models them. When a pressure reducing valve is open, the head loss across the valve will be equal to the pressure reduction, which is encoded as a constant parameter associated with the valve. Flow is only constraint by nonnegativity across a pressure reducing valve.

[tasseff]

Correction, when a pressure reducing valve is open, the pressure at the downstream node will be equal to the (constant) setting parameter associated with the valve. Additionally, negative head differences are disallowed when the pressure reducing valve is open.