Save retainer debug state so it isn't lost during load from savegame

Author: SteelFill

Source/Orts.Simulation/Simulation/RollingStocks/SubSystems/Brakes/MSTS/AirSinglePipe.cs

@@ -321,6 +321,7 @@ public override void Save(BinaryWriter outf)
             outf.Write(HandbrakePercent);
             outf.Write(ReleaseRatePSIpS);
             outf.Write(RetainerPressureThresholdPSI);
+            outf.Write(RetainerDebugState);
             outf.Write(AutoCylPressurePSI);
             outf.Write(AuxResPressurePSI);
             outf.Write(EmergResPressurePSI);
@@ -344,6 +345,7 @@ public override void Restore(BinaryReader inf)
             HandbrakePercent = inf.ReadSingle();
             ReleaseRatePSIpS = inf.ReadSingle();
             RetainerPressureThresholdPSI = inf.ReadSingle();
+            RetainerDebugState = inf.ReadString();
             AutoCylPressurePSI = inf.ReadSingle();
             AuxResPressurePSI = inf.ReadSingle();
             EmergResPressurePSI = inf.ReadSingle();