N-dimensional time series

[nmheim]

It would be great if Rodent would support n-dimensional timeseries. However, this raises some issues with our definition of xlength and the computation of the jacobian of the decoder.

[nmheim]

replace xlength/zlength with tuple xsize/zsize

[nmheim]

We will remove xlenth/xsize all toghether, so this will need a custom type of CGaussian with Kronecker product variance.