Varonoi partitioning with few points generates empty areas

[davide-f]

Dear developer,

when working at the issue pypsa-meets-earth/pypsa-earth#99 , I noticed that when applying the voronoi partitioning with few points in the shape the result was the one shown below.

That was due to the limits in the size of the voronoi shape that was limited to the size of the points (with a multiplier), while the outline shape was not considered.

Shortly, I post a Pull request to solve the issue.