Support for 3D tensors

[sloede]

It would be great if we could also support 3D tensors besides vectors and matrices, e.g., to be able to do 3D simulations. This should probably be done after #51 though, since a single ciphertext with ring dimension $N = 2^{17}$ can only hold tensors up to approximately $40 \times 40 \times 40$.