Example of huge improvement with lists of polygons

A trivial example of plotting of a list of polygons with the original
way and with the new list of polygons separated by numpy.nan. Note that
for the original method the computational time grows exponentially
(3.8s for 1000, 14.1s for 2000 on my machine) with the number of
polygons while the new is mostly insensitive (0.007 for 1000, 0.008 for
2000).