code: private Vector3 middleOfCell(int x_index, int y_index) {
          float middleXCell = (float) (indexRegion.minX() + x_index * x_size + x_size / 2);
          float middleYCell = (float) (indexRegion.minY() + y_index * y_size + y_size / 2);
          return new Vector3(middleXCell, middleYCell, 0);
      }
docs: Create a point at the center of a cell
  
  @param x_index horizontal cell index
  @param y_index vertical cell index
  @return Point at the center of the cell at (x_index, y_index)