Simulator: reduce default rate

For lockstep we use a default rate of 250 Hz. Therefore, it makes sense
to use this as a deafult rate as well. That way we can start it without
passing on any options and it will work out of the box.

src/me/drton/jmavsim/Simulator.java

@@ -53,7 +53,7 @@ private static enum Port {
         true;   // send System.out messages to stdout (console) as well as any custom handlers (see SystemOutHandler)
     public static boolean DEBUG_MODE = false;
 
-    public static final int    DEFAULT_SIM_RATE = 500; // Hz
+    public static final int    DEFAULT_SIM_RATE = 250; // Hz
     public static final double    DEFAULT_SPEED_FACTOR = 1.0;
     public static final int    DEFAULT_AUTOPILOT_SYSID =
         -1; // System ID of autopilot to communicate with. -1 to auto set ID on first received heartbeat.