AP_Rangefinder: request measurement for timer routine to collect

the timer routing attempts to read a reading off the bus straihgt-up, but after initialisation the sensor will not be attempting to provide one, so this will always fail.

Request a sample as the last thing we do in the init routine

libraries/AP_RangeFinder/AP_RangeFinder_TeraRangerI2C.cpp

@@ -97,6 +97,9 @@ bool AP_RangeFinder_TeraRangerI2C::init(void)
         return false;
     }
 
+    // ask for a new reading for the timer to collect:
+    measure();
+
     dev->get_semaphore()->give();
 
     dev->set_retries(1);