ArduPlane: rename OpticalFlow class to AP_OpticalFlow

Brings us in-line with other classes in ArduPilot.

Removes ambiguity with AP_HAL::OpticalFlow which can cause compilation errors as we start to make code more portable across targets

ArduPlane/ArduPlane.cpp

@@ -83,7 +83,7 @@ const AP_Scheduler::Task Plane::scheduler_tasks[] = {
     SCHED_TASK_CLASS(AP_ICEngine,      &plane.g2.ice_control, update,     10, 100,  81),
 #endif
 #if AP_OPTICALFLOW_ENABLED
-    SCHED_TASK_CLASS(OpticalFlow, &plane.optflow, update,    50,    50,  87),
+    SCHED_TASK_CLASS(AP_OpticalFlow, &plane.optflow, update,    50,    50,  87),
 #endif
     SCHED_TASK(one_second_loop,         1,    400,  90),
     SCHED_TASK(three_hz_loop,           3,     75,  93),

ArduPlane/GCS_Plane.cpp

@@ -99,7 +99,7 @@ void GCS_Plane::update_vehicle_sensor_status_flags(void)
     }
 
 #if AP_OPTICALFLOW_ENABLED
-    const OpticalFlow *optflow = AP::opticalflow();
+    const AP_OpticalFlow *optflow = AP::opticalflow();
     if (optflow && optflow->enabled()) {
         control_sensors_present |= MAV_SYS_STATUS_SENSOR_OPTICAL_FLOW;
         control_sensors_enabled |= MAV_SYS_STATUS_SENSOR_OPTICAL_FLOW;

ArduPlane/Parameters.cpp

@@ -939,7 +939,7 @@ const AP_Param::Info Plane::var_info[] = {
 #if AP_OPTICALFLOW_ENABLED
     // @Group: FLOW
     // @Path: ../libraries/AP_OpticalFlow/AP_OpticalFlow.cpp
-    GOBJECT(optflow,   "FLOW", OpticalFlow),
+    GOBJECT(optflow,   "FLOW", AP_OpticalFlow),
 #endif
 
     // @Group: MIS_

ArduPlane/Plane.h

@@ -240,7 +240,7 @@ private:
 
 #if AP_OPTICALFLOW_ENABLED
     // Optical flow sensor
-    OpticalFlow optflow;
+    AP_OpticalFlow optflow;
 #endif
 
     // Rally Ponints