code: private Vector2i getCellOf(Point3D coordinate) {
          int xCell = (int) ((coordinate.x - indexRegion.minX()) / x_size);
          int yCell = (int) ((coordinate.y - indexRegion.minY()) / y_size);
          return new Vector2i(xCell, yCell);
      }
docs: Locates the grid cell point covering the given coordinate
  
  @param coordinate world coordinate to locate
  @return cell covering the coordinate