Add type checking for img detection messages

[jkbmrz]

Adding type checking for keypoints and detections attributes for ImgDetectionWithKeypoints and ImgDetectionsWithKeypoints messages using Python descriptors: keypoints must be an empty list or a list of tuples of two floats or integers; detections must be ImgDetectionWithKeypoints object.

[tersekmatija]

Looks OK, but not sure if we actually need descriptor classes given we are not reusing them anywhere else?

Should we just handle validation in setter/getter methods?