AC_Sprayer: add consts, update comments

* removed some comments that were left over from the classes this class
was copypasta'ed from

libraries/AC_Sprayer/AC_Sprayer.cpp

@@ -50,8 +50,7 @@ const AP_Param::GroupInfo AC_Sprayer::var_info[] PROGMEM = {
     AP_GROUPEND
 };
 
-/// Default constructor.
-AC_Sprayer::AC_Sprayer(AP_InertialNav* inav) :
+AC_Sprayer::AC_Sprayer(const AP_InertialNav* inav) :
     _inav(inav),
     _speed_over_min_time(0),
     _speed_under_min_time(0)
@@ -69,7 +68,6 @@ AC_Sprayer::AC_Sprayer(AP_InertialNav* inav) :
     // To-Do: ensure that the pump and spinner servo channels are enabled
 }
 
-/// enable - allows fence to be enabled/disabled.  Note: this does not update the eeprom saved value
 void AC_Sprayer::enable(bool true_false)
 {
     // return immediately if no change

libraries/AC_Sprayer/AC_Sprayer.h

@@ -34,19 +34,17 @@
 #define AC_SPRAYER_DEFAULT_TURN_ON_DELAY    100     // delay between when we reach the minimum speed and we begin spraying.  This reduces the likelihood of constantly turning on/off the pump
 #define AC_SPRAYER_DEFAULT_SHUT_OFF_DELAY   1000    // shut-off delay in milli seconds.  This reduces the likelihood of constantly turning on/off the pump
 
-/// @class	Camera
-/// @brief	Object managing a Photo or video camera
 class AC_Sprayer {
 
 public:
 
     /// Constructor
-    AC_Sprayer(AP_InertialNav* inav);
+    AC_Sprayer(const AP_InertialNav* inav);
 
     /// enable - allows sprayer to be enabled/disabled.  Note: this does not update the eeprom saved value
     void enable(bool true_false);
 
-    /// enabled - returns true if fence is enabled
+    /// enabled - returns true if sprayer is enabled
     bool enabled() const { return _enabled; }
 
     /// test_pump - set to true to turn on pump as if travelling at 1m/s as a test
@@ -64,7 +62,7 @@ public:
 
 private:
     // pointers to other objects we depend upon
-    AP_InertialNav* _inav;
+    const AP_InertialNav* const _inav;
 
     // parameters
     AP_Int8         _enabled;               // top level enable/disable control