Two modification for easy usage.

[jarvisyjw]

Add functions:

1. Add a PDF format calibration target with an absolute scale for easy printing.
2. Add direct image extraction (extract the first frame a the given image topic) from rosbag if the provided image_path returns an empty image. This process is compatible with the existing code. If a valid image_path is provided, the code won't extract an image from the rosbag.

Required dependencies:

1. cv_bridge
2. image_transport