Refactor image classification get_classes().
Refactor image classification get_classes().

Add object_prob_threshold to restrict probabilities, change
max_num_objects default to return all detected objects. Default
behaviour now is to return all detected classes with non-zero
probabilities.

Change-Id: Ibbf97d04dc609bf25c0d5329b7757d1391c9b7cf

google/aiyprojects-raspbian,google/aiyprojects-raspbian,google/aiyprojects-raspbian,google/aiyprojects-raspbian,google/aiyprojects-raspbian