[Refactor] separate image message & OpenCV image classification, use dictionary for classes

[minhnh]

Changelog

• break the classify function into classify and classify_np_images, where the latter may classify a list of OpenCV images and the former a list of sensor_msgs/Image messages
• use dictionary to keep track of classes in ImageClassifier, update relevant function and documentation

Motivation

• allow for classifying OpenCV images directly, which is needed by Refactor mdr_find_people mas_domestic_robotics#152 for cutout face and body images
• make keeping track of classes for ImageClassifier consistent with ImageDetector and more flexible (e.g. allow for skipping classes)

QA

• tested locally with the object recognition service and corresponding test script

roslaunch mdr_object_recognition object_recognition.launch
rosrun mas_perception_libs -t ${IMAGE_DIR} -s ${SERVICE_NAME} ${MODEL_NAME}