Compass: fix compile warnings re float constants

Also fix example sketch

libraries/AP_Compass/AP_Compass_HMC5843.cpp

@@ -236,7 +236,7 @@ AP_Compass_HMC5843::init()
          */
         expected_x = 766;
         expected_yz  = 713;
-        gain_multiple = 660.0 / 1090;  // adjustment for runtime vs calibration gain
+        gain_multiple = 660.0f / 1090;  // adjustment for runtime vs calibration gain
     } else if (_base_config == (NormalOperation | DataOutputRate_75HZ<<2)) {
         _product_id = AP_COMPASS_TYPE_HMC5843;
     } else {

libraries/AP_Compass/Compass_learn.cpp

@@ -34,9 +34,9 @@ Compass::learn_offsets(void)
 
     // this gain is set so we converge on the offsets in about 5
     // minutes with a 10Hz compass
-    const float gain = 0.01;
-    const float max_change = 10.0;
-    const float min_diff = 50.0;
+    const float gain = 0.01f;
+    const float max_change = 10.0f;
+    const float min_diff = 50.0f;
 
     if (!_null_init_done) {
         // first time through

libraries/AP_Compass/examples/AP_Compass_test/AP_Compass_test.pde

@@ -53,7 +53,7 @@ void setup() {
     hal.console->printf("init done - %u compasses detected\n", compass.get_count());
 
     compass.set_and_save_offsets(0,0,0,0); // set offsets to account for surrounding interference
-    compass.set_declination(ToRad(0.0)); // set local difference between magnetic north and true north
+    compass.set_declination(ToRad(0.0f)); // set local difference between magnetic north and true north
 
     hal.scheduler->delay(1000);
     timer = hal.scheduler->micros();