Parametric NACA4 foil

An even cleaner and more parametric remix of the NACA4 airfoil. The number of points is now also a parameter, requiring list comprehension for the polygon (this also improves readability). The trailing edge can optionally be closed. The module was renamed to naca4 to avoid confusion ; the default value for the new parameters lead to the same airfoil as in the original module.
The duct fan OpenSCAD file is an example of what can be done with the parametric NACA4 foil generator ; it includes hollow arm, hollow duct and reinforcing ribs inside them, all obtained through NACA4 and some offset / scale.