let PressureLoss implement IObservable interface rather than Observer

refs #334

src/pipe_flow/pressureloss.ts

@@ -6,7 +6,7 @@ import { PressureLossParams } from "./pressureloss_params";
 import { PressureLossLaw } from "../pipe_flow/pressureloss_law";
 import { PressureLossLawParams } from "../pipe_flow/pressurelosslaw_params";
 
-export class PressureLoss extends Nub implements Observer {
+export class PressureLoss extends Nub {
 
     constructor(law: PressureLossLaw, dbg: boolean = false) {
         super(new PressureLossParams(), dbg);
@@ -43,8 +43,13 @@ export class PressureLoss extends Nub implements Observer {
         return this.child.Equation(sVarCalc);
     }
 
-    // interface Observer
+    // interface IObservable
 
-    public update(sender: any, data: any) {
+    public addObserver(o: Observer) {
+        this.child.addObserver(o);
+    }
+
+    public removeObserver(o: Observer) {
+        this.child.removeObserver(o);
     }
 }