Fixing empty areas with low number of points

[davide-f]

This pull request solve the issue #16.

The result of the partitioning of the problem described in #16 is shown below.

[pz-max]

I like that you added the multiplier as a variable, which keeps the function a bit more flexible when the data situation is limited. Could you explain the purpose of the add_bounds_shape in more detail? :)

voronoi_partition_pts(points, outline, no_multipolygons=False, add_bounds_shape=True, multiplier=5):

[davide-f]

Thanks @pz-max for the suggestion; I forgot to update the description, which was instead needed.