Have webcams destroy windows they own when exited

Before, webcams did not manage their own windows, so although they released
their cameras on exit, the windows remained open to be closed manually. This is
no longer the case.
Extract calibration into top-level function

This enables the calibration function to be called from other files as well, so
that I can add the option of live calibration to ``chessboard_photos.py``. The
function is also called from ``main`` in ``calibrate_stereo.py``, so it's
unredundant.
Move argparse parent to ``calibrate_stereo.py``

I am planning on using ``calibrate_folder`` in ``chessboard_photos.py`` and
want to avoid a circular import.
Allow user to calibrate stereo pair live

This is done by using ``calibrate_folder`` from ``calibrate_stereo.py``.