AP_Compass: don't reset SITL compass offsets if set by user

makes for easier EKF compass bias learning testing

libraries/AP_Compass/AP_Compass_SITL.cpp

@@ -12,7 +12,9 @@ AP_Compass_SITL::AP_Compass_SITL()
         _compass._setup_earth_field();
         for (uint8_t i=0; i<SITL_NUM_COMPASSES; i++) {
             // default offsets to correct value
-            _compass.set_offsets(i, _sitl->mag_ofs);
+            if (_compass.get_offsets(i).is_zero()) {
+                _compass.set_offsets(i, _sitl->mag_ofs);
+            }
             
             _compass_instance[i] = register_compass();
             set_dev_id(_compass_instance[i], AP_HAL::Device::make_bus_id(AP_HAL::Device::BUS_TYPE_SITL, i, 0, DEVTYPE_SITL));