Add a CLI setup action to reset the magnetometer offsets

Since the magnetometer offsets are not available through the MAVLink parameter interface (since they are an AP_Var vector) this little feature allows them to be reset from the CLI.  Useful if you somehow get bad offsets or if you change magnetometer.  If you have a bad set of large offset values I have seen issues with the nulling algorithm have trouble converging to the proper values.  I have never seen it have trouble converging from 0/0/0, so this could be a useful feature from time to time.

ArduPlane/setup.pde

@@ -301,8 +301,11 @@ setup_compass(uint8_t argc, const Menu::arg *argv)
 	} else if (!strcmp_P(argv[1].str, PSTR("off"))) {
 		g.compass_enabled = false;
 
+	} else if (!strcmp_P(argv[1].str, PSTR("reset"))) {
+		compass.set_offsets(0,0,0);
+
 	} else {
-		Serial.printf_P(PSTR("\nOptions:[on,off]\n"));
+		Serial.printf_P(PSTR("\nOptions:[on,off,reset]\n"));
 		report_compass();
 		return 0;
 	}

libraries/AP_Compass/Compass.h

@@ -102,8 +102,6 @@ public:
 
 	/// Program new offset values.
 	///
-	/// XXX DEPRECATED
-	///
 	/// @param  x                   Offset to the raw mag_x value.
 	/// @param  y                   Offset to the raw mag_y value.
 	/// @param  z                   Offset to the raw mag_z value.