The function draw_pm sends an object of the template parameter
type Planar_map to a graphic stream. The function is intended
for cases in which the drawing object is not the default one, i.e. for
user defined drawing classes. User defined drawers aim at drawing
planar maps with additional information or aim at a different way to
draw components. For a default use simply use the insertor operator
(<<) of the graphic stream as in my_window << pm.
