AP_Gripper: a valid() method

libraries/AP_Gripper/AP_Gripper.cpp

@@ -125,6 +125,7 @@ PASS_TO_BACKEND(update)
         return false;                         \
     }
 
+PASS_TO_BACKEND(valid)
 PASS_TO_BACKEND(released)
 PASS_TO_BACKEND(grabbed)
 

libraries/AP_Gripper/AP_Gripper.h

@@ -44,6 +44,9 @@ public:
     // update - should be called at at least 10hz
     void update();
 
+    // valid - returns true if we have a gripper and it should work
+    bool valid() const;
+
     static const struct AP_Param::GroupInfo        var_info[];
 
     // parameters

libraries/AP_Gripper/AP_Gripper_Backend.h

@@ -34,6 +34,9 @@ public:
     // release - move the servo output to the release position
     virtual void release() = 0;
 
+    // valid - returns true if the backend should be working
+    bool valid() const { return true; };
+
     // released - returns true if currently in released position
     virtual bool released() const = 0;