PID: standardize inclusion of libaries headers

This commit changes the way libraries headers are included in source files:

 - If the header is in the same directory the source belongs to, so the
 notation '#include ""' is used with the path relative to the directory
 containing the source.

 - If the header is outside the directory containing the source, then we use
 the notation '#include <>' with the path relative to libraries folder.

Some of the advantages of such approach:

 - Only one search path for libraries headers.

 - OSs like Windows may have a better lookup time.

libraries/PID/PID.cpp

@@ -6,8 +6,8 @@
 #include <math.h>
 
 #include "PID.h"
-#include <AP_HAL.h>
-#include <AP_Math.h>
+#include <AP_HAL/AP_HAL.h>
+#include <AP_Math/AP_Math.h>
 
 extern const AP_HAL::HAL& hal;
 

libraries/PID/PID.h

@@ -6,8 +6,8 @@
 #ifndef __PID_H__
 #define __PID_H__
 
-#include <AP_Common.h>
-#include <AP_Param.h>
+#include <AP_Common/AP_Common.h>
+#include <AP_Param/AP_Param.h>
 #include <stdlib.h>
 #include <math.h>               // for fabs()
 

libraries/PID/examples/pid/pid.cpp

@@ -3,17 +3,17 @@
  *       2010 Code by Jason Short. DIYDrones.com
  */
 
-#include <AP_HAL.h>
-#include <AP_Common.h>
-#include <AP_Progmem.h>
-#include <AP_Param.h>
-#include <StorageManager.h>
-#include <AP_Math.h>
-#include <PID.h> // ArduPilot Mega RC Library
+#include <AP_HAL/AP_HAL.h>
+#include <AP_Common/AP_Common.h>
+#include <AP_Progmem/AP_Progmem.h>
+#include <AP_Param/AP_Param.h>
+#include <StorageManager/StorageManager.h>
+#include <AP_Math/AP_Math.h>
+#include <PID/PID.h> // ArduPilot Mega RC Library
 
-#include <AP_HAL_AVR.h>
-#include <AP_HAL_SITL.h>
-#include <AP_HAL_Empty.h>
+#include <AP_HAL_AVR/AP_HAL_AVR.h>
+#include <AP_HAL_SITL/AP_HAL_SITL.h>
+#include <AP_HAL_Empty/AP_HAL_Empty.h>
 
 const AP_HAL::HAL& hal = AP_HAL_BOARD_DRIVER;