AP_Scheduler: added ticks32() API

this is the number of scheduler ticks since boot, as uint32_t

libraries/AP_Scheduler/AP_Scheduler.cpp

@@ -154,6 +154,7 @@ void AP_Scheduler::init(const AP_Scheduler::Task *tasks, uint8_t num_tasks, uint
 void AP_Scheduler::tick(void)
 {
     _tick_counter++;
+    _tick_counter32++;
 }
 
 #if CONFIG_HAL_BOARD == HAL_BOARD_SITL

libraries/AP_Scheduler/AP_Scheduler.h

@@ -123,6 +123,7 @@ public:
 
     // return current tick counter
     uint16_t ticks() const { return _tick_counter; }
+    uint32_t ticks32() const { return _tick_counter32; }
 
     // run the tasks. Call this once per 'tick'.
     // time_available is the amount of time available to run
@@ -224,6 +225,7 @@ private:
     // number of 'ticks' that have passed (number of times that
     // tick() has been called
     uint16_t _tick_counter;
+    uint32_t _tick_counter32;
 
     // tick counter at the time we last ran each task
     uint16_t *_last_run;