StereoCalibration creates export folder if neeeded

This makes a ``StereoCalibration`` behave more robustly: If you ask it to
export itself to a nonexistent folder, it creates the folder. Previously, this
led to an exception.
Cleanup chessboard_photos.py's UI

If a user specifies a calibration folder, he obviously wants to calibrate.
Therefore there was no purpose in forcing the user to use the calibration flag
when he wanted to calibrate. Now that option has been removed, simplifying the
UI. Also, unused imports were cleaned up.
Adjust available StereoBM settings

Previously, only very conservative settings were allowed, and the GUI did not
restrict these settings dynamically. Now a bit of redundancy has been removed -
the GUI looks at what the maximum settings are, and they're stored on the class
themselves, rather than using the same literal twice in the same file.
Normalize disparity map in GUI

This makes it a lot easier to tune the StereoBM.