AC_PID: expose ff() method in AC_PID

this allows for an abstract AC_PID class to be used in
AC_AttitudeControl for both multicopter and heli

libraries/AC_PID/AC_HELI_PID.h

@@ -25,14 +25,9 @@ public:
     /// get_leaky_i - replacement for get_i but output is leaded at leak_rate
     float       get_leaky_i(float leak_rate);
 
-    // accessors
-    float       ff() const { return _vff.get(); }
-    void        ff(const float v) { _vff.set(v); }
-
     static const struct AP_Param::GroupInfo        var_info[];
 
 private:
-    AP_Float        _vff;
     AP_Float        _leak_min;
 
     float           _last_requested_rate;       // Requested rate from last iteration, used to calculate rate change of requested rate

libraries/AC_PID/AC_PID.h

@@ -62,6 +62,7 @@ public:
     float       imax() const { return _imax.get(); }
     float       filt_hz() const { return _filt_hz.get(); }
     float       get_filt_alpha() const;
+    float       ff() const { return _vff.get(); }
 
     // set accessors
     void        kP(const float v) { _kp.set(v); }
@@ -69,6 +70,7 @@ public:
     void        kD(const float v) { _kd.set(v); }
     void        imax(const float v) { _imax.set(fabsf(v)); }
     void        filt_hz(const float v);
+    void        ff(const float v) { _vff.set(v); }
 
     float       get_integrator() const { return _integrator; }
     void        set_integrator(float i) { _integrator = i; }
@@ -89,6 +91,7 @@ protected:
     AP_Float        _kd;
     AP_Float        _imax;
     AP_Float        _filt_hz;                   // PID Input filter frequency in Hz
+    AP_Float        _vff; // only used by heli
 
     // flags
     struct ac_pid_flags {