格子のセルの面積を計算して返す。
call cg_iric_read_grid2d_cellarea(g_handle, cell_id, area, ier)
ier = cg_iRIC_Read_Grid2d_CellArea(g_handle, cell_id, &area);
area = cg_iRIC_Read_Grid2d_CellArea(g_handle, cell_id)