The cells are designed so they can be set up side by side in modular construction. In order to save material and costs, a wall of the one cell is placed side by side, with the wall of the other cell. This results in the typical A, B and C cells, in which the A-cell has four walls, to which a B-cell, with only 3 walls can be screwed. In the case of two or more rows, these cells are supplemented by the C-cell with only two walls.