Default thresholds for ground sensors

Created by: motib

In the transition from 1.3 to 1.4, the default thresholds for the ground sensors have been changed from 150 and 300 to 350 and 450. When resting on a light-colored table, one sensor on my Thymio returns only about 420, while on a black background or when "in the air", the sensors return values less that 100. These readings are consistent with the old thresholds and I request that they be restored.