Feature/image to world transformation

The python variant of the StereoCalibrationInterface and camera_utils.py provide less functionality than the C++ Calibration class. To partially address that, namely for the conversion between image and world coordinates, this merge request suggests two additional functions (following the current code style) and a class (being able to re-use the initialization, which includes retrieving the calibration via a proxy, and thus being more convenient and probably faster).