AP_Scheduler: remove loop-period argument from load_average

This now comes from a parameter rather than being hard-set
in the vehicle code

Also adds a method returning the max loop period based on the
loop rate

libraries/AP_Scheduler/AP_Scheduler.cpp

@@ -189,11 +189,12 @@ uint16_t AP_Scheduler::time_available_usec(void)
 /*
   calculate load average as a number from 0 to 1
  */
-float AP_Scheduler::load_average(uint32_t tick_time_usec) const
+float AP_Scheduler::load_average() const
 {
     if (_spare_ticks == 0) {
         return 0.0f;
     }
-    uint32_t used_time = tick_time_usec - (_spare_micros/_spare_ticks);
-    return used_time / (float)tick_time_usec;
+    const uint32_t loop_us = get_loop_period_us();
+    const uint32_t used_time = loop_us - (_spare_micros/_spare_ticks);
+    return used_time / (float)loop_us;
 }

libraries/AP_Scheduler/AP_Scheduler.h

@@ -83,12 +83,16 @@ public:
     // return load average, as a number between 0 and 1. 1 means
     // 100% load. Calculated from how much spare time we have at the
     // end of a run()
-    float load_average(uint32_t tick_time_usec) const;
+    float load_average() const;
 
     // get the configured main loop rate
     uint16_t get_loop_rate_hz(void) const {
         return _loop_rate_hz;
     }
+    // get the time-allowed-per-loop:
+    uint32_t get_loop_period_us() const {
+        return 1000000UL / _loop_rate_hz;
+    }
 
     static const struct AP_Param::GroupInfo var_info[];