n and the connecting strips 3 1 3 2 3 n are produced from the same type of sheet material as the strips 2 1 2 2 2 n the connecting strips 3 1 3 2 3 n can likewise be formed with multiple layers if for example the connecting strips 3 1 3 2 3 n are formed in an identical way to the strips 2 1 2 2 2 n as described above the result that follows after combination of the connecting strips 3 1 3 2 3 n with the strips 2 1 2 2 2 n is an overall four layer material it is likewise possible not to arrange the connecting strips 3 1 3 2 3 n between the two layers of the strips 2 1 2 2 2 n but on or under the strips 2 1 2 2 2 n and in each case such that a continuous surface is formed in a further conceivable embodiment provision can be made for the connecting strips 3 1 3 2 3 n to have a width which is lower than the width of the strips 2 1 2 2 2 n as a result the overlap between the strips 2 1 2 2 2 n and the connecting strips 3 1 3.