contains method
Returns true if point (x, y) is inside the Voronoi cell for the given cellPoint.
Implementation
bool contains(Point cellPoint, double x, double y) {
final i = _findNearestPoint(x, y);
return i >= 0 &&
delaunay.getX(i) == cellPoint.x &&
delaunay.getY(i) == cellPoint.y;
}