Cliff sensor modifications

[stonier]

Conversion

Currently Kobuki outputs a human-_un_friendly measurement for the distance to the floor in case a cliff is detected (see kobuki_msgs/CliffEvent -> bottom).

The current output is the A/D-value of the sensor (Integer, 0 - 4095) and doesn't allow the users to related to an actual distance (e.g. in meters).

I suggest we add a conversion to the driver to output the mesured distance in meter.

Threshold

Currently thresholded at 8cm in firmware, too much for turtlebot. 4cm better.

Details

Conversion : put a lookup table in kobuki driver and publish both analog and converted values (converted will lose accuracy)