Correct buggy behavior in ``UltrasonicGroundSensors``

1. The deviance was being measured in signed numbers, and I was checking if the
deviance *exceeded* the maximum deviance. This of course ignored negative
numbers, so if you walked over stairs leading down you wouldn't sense them.

2. The wrong function call was being used - ``max`` rather than
``max_element``, analog for ``min``. I was comparing the pointers returned by
``cbegin()`` and ``cend()``, rather than iterating through the elements between
and comparing their values. This mistake has been removed.