FIX Circularise bug. There was an odd bug that caused the centroid to be calculated...

FIX Circularise bug. There was an odd bug that caused the centroid to be calculated incorrectly, yielding oversized, misplaced circles. Inspired by openstreetmap.org/user/alexz/diary/13054, I replace the centroid calculation algorithm with a much simpler "average of X and Y". In my testing, it yields good results. It's technically "incorrect" for heavily skewed polygons, but IMHO there the result is still acceptable, and it's hardly what Circularise is designed for.