AC_PID: example fix travis warning

missing function declaration
implicit cast
some style fix

libraries/AC_PID/examples/AC_PID_test/AC_PID_test.cpp

@@ -7,6 +7,9 @@
 #include <AC_PID/AC_PID.h>
 #include <AC_PID/AC_HELI_PID.h>
 
+void setup();
+void loop();
+
 const AP_HAL::HAL& hal = AP_HAL::get_HAL();
 
 // default PID values
@@ -38,12 +41,12 @@ void loop()
     float control_P, control_I, control_D;
 
     // display PID gains
-    hal.console->printf("P %f  I %f  D %f  imax %f\n", (float)pid.kP(), (float)pid.kI(), (float)pid.kD(), (float)pid.imax());
+    hal.console->printf("P %f  I %f  D %f  imax %f\n", (double)pid.kP(), (double)pid.kI(), (double)pid.kD(), (double)pid.imax());
 
     // capture radio trim
     radio_trim = hal.rcin->read(0);
 
-    while( true ) {
+    while (true) {
         radio_in = hal.rcin->read(0);
         error = radio_in - radio_trim;
         pid.set_input_filter_all(error);
@@ -54,8 +57,8 @@ void loop()
         // display pid results
         hal.console->printf("radio: %d\t err: %d\t pid:%4.2f (p:%4.2f i:%4.2f d:%4.2f)\n",
                 (int)radio_in, (int)error,
-                (float)(control_P+control_I+control_D),
-                (float)control_P, (float)control_I, (float)control_D);
+                (double)(control_P+control_I+control_D),
+                (double)control_P, (double)control_I, (double)control_D);
         hal.scheduler->delay(50);
     }
 }