AP_HAL_SITL: added SIM_GPS_ALT_OFS

this is used to give a bad GPS height in SITL, which is very useful for
testing origin vs home issues

libraries/AP_HAL_SITL/sitl_gps.cpp

@@ -1126,6 +1126,8 @@ void SITL_State::_update_gps(double latitude, double longitude, float altitude,
     if (AP_HAL::millis() < _sitl->gps_lock_time*1000UL) {
         have_lock = false;
     }
+
+    altitude += _sitl->gps_alt_offset;
     
     //Capture current position as basestation location for
     if (!_gps_has_basestation_position) {