boxspring_figure.py is missing

The module referred from `boxspring_figure_anim.py` is missing:
```
# make sure that we find the boxspring_figure module in ../oo:
#sys.path.insert(0, '../oo')
# better:
sys.path.insert(0, os.path.join(os.pardir, 'oo'))
from boxspring_figure import draw, set_figure_size
```