PID: fix compile warnings re float constants

libraries/PID/PID.h

@@ -16,9 +16,9 @@
 class PID {
 public:
 
-    PID(const float &   initial_p = 0.0,
-        const float &   initial_i = 0.0,
-        const float &   initial_d = 0.0,
+    PID(const float &   initial_p = 0.0f,
+        const float &   initial_i = 0.0f,
+        const float &   initial_d = 0.0f,
         const int16_t & initial_imax = 0)
     {
 		AP_Param::setup_object_defaults(this, var_info);

libraries/PID/examples/pid/pid.pde

@@ -32,7 +32,7 @@ void setup()
 
     pid.kP(1);
     pid.kI(0);
-    pid.kD(0.5);
+    pid.kD(0.5f);
     pid.imax(50);
     pid.save_gains();
     pid.kP(0);