Put AMPL generation code in separate file

The amount of code was getting a bit much to leave this inside the
`NebulousApp` class.

Change-Id: I31228c904a9b458d4cf90829d73684399d9cc655

optimiser-controller/src/main/java/eu/nebulouscloud/optimiser/controller/LocalExecution.java

@@ -52,7 +52,7 @@ public class LocalExecution implements Callable<Integer> {
             log.info("Sending AMPL to channel {}", publisher);
             app.sendAMPL();
         }
-        System.out.println(app.generateAMPL());
+        System.out.println(AMPLGenerator.generateAMPL(app));
         // TODO: wait for solver reply here?
         return 0;
     }