AP_Scheduler: allow registration of tasks at loop rate

libraries/AP_Scheduler/AP_Scheduler.cpp

@@ -160,7 +160,8 @@ void AP_Scheduler::run(uint32_t time_available)
         const AP_Scheduler::Task& task = (i < _num_unshared_tasks) ? _tasks[i] : _common_tasks[i - _num_unshared_tasks];
 
         uint32_t dt = _tick_counter - _last_run[i];
-        uint32_t interval_ticks = _loop_rate_hz / task.rate_hz;
+        // we allow 0 to mean loop rate
+        uint32_t interval_ticks = (is_zero(task.rate_hz) ? 1 : _loop_rate_hz / task.rate_hz);
         if (interval_ticks < 1) {
             interval_ticks = 1;
         }

libraries/AP_Scheduler/AP_Scheduler.h

@@ -26,6 +26,7 @@
 #include "PerfInfo.h"       // loop perf monitoring
 
 #define AP_SCHEDULER_NAME_INITIALIZER(_name) .name = #_name,
+#define LOOP_RATE 0
 
 /*
   useful macro for creating scheduler task table