AP_HAL_SITL: use method for downcast

Instead of just doing a static cast to the desired class, use a method
named "from". Pros:

  - When we have data shared on the parent class, the code is cleaner in
    child class when it needs to access this data. Almost all the data
    we use in AP_HAL benefits from this

  - There's a minimal type checking because now we are using a method
    that can only receive the type of the parent class

libraries/AP_HAL_SITL/SITL_State.cpp

@@ -524,7 +524,7 @@ void SITL_State::init(int argc, char * const argv[])
     pwm_input[4] = pwm_input[7] = 1800;
     pwm_input[2] = pwm_input[5] = pwm_input[6] = 1000;
 
-    _scheduler = (SITLScheduler *)hal.scheduler;
+    _scheduler = SITLScheduler::from(hal.scheduler);
     _parse_command_line(argc, argv);
 }
 

libraries/AP_HAL_SITL/Scheduler.h

@@ -13,6 +13,10 @@
 class HALSITL::SITLScheduler : public AP_HAL::Scheduler {
 public:
     SITLScheduler(SITL_State *sitlState);
+    static SITLScheduler *from(AP_HAL::Scheduler *scheduler) {
+        return static_cast<SITLScheduler*>(scheduler);
+    }
+
     /* AP_HAL::Scheduler methods */
 
     void     init(void *unused);