shape_predictor: allow dpoint as training input

Currently, the shape predictor training example [showcases how to provide](https://github.com/davisking/dlib/blob/master/examples/faces/training_with_face_landmarks.xml) training and test data using integer coordinates.

Hoever, I would like to train a shape predictor using float coordinate pairs. I hope to gain increased precision by doing so. Since the shape predictor is also giving float numbers as output, I would assume that this should be possible.

However, there is no documentation to be found on the input format.

I'm happy to extend the documentation as soon as I know how this works.