AHRS_DCM: fix compile warnings re float constants

Also fix example sketch

libraries/AP_AHRS/AP_AHRS_DCM.cpp

@@ -268,7 +268,7 @@ AP_AHRS_DCM::yaw_error_compass(void)
     rb.normalize();
     if (rb.is_inf()) {
         // not a valid vector
-        return 0.0;
+        return 0.0f;
     }
 
     // update vector holding earths magnetic field (if required)

libraries/AP_AHRS/AP_AHRS_DCM.h

@@ -55,8 +55,8 @@ public:
 
         // these are experimentally derived from the simulator
         // with large drift levels
-        _ki = 0.0087;
-        _ki_yaw = 0.01;
+        _ki = 0.0087f;
+        _ki_yaw = 0.01f;
     }
 
     // return the smoothed gyro vector corrected for drift

libraries/AP_AHRS/examples/AHRS_Test/AHRS_Test.pde

@@ -124,7 +124,7 @@ void loop(void)
                         ToDeg(drift.x),
                         ToDeg(drift.y),
                         ToDeg(drift.z),
-                        compass.use_for_yaw() ? ToDeg(heading) : 0.0,
+                        compass.use_for_yaw() ? ToDeg(heading) : 0.0f,
                         (1.0e6*counter)/(now-last_print));
         last_print = now;
         counter = 0;