SLH block-feedback

[goerz]

Since we have worked out the SLH rule for applying feedback to a whole block of output channels at once, this should be implemented in the circuit_algebra module.

We might also want to lift the restriction that Feedback can only happen from the bottom output channel (this adds needless permutations that make the resulting SLH expressions more complicated).

The printing and circuit visualization will also have to be modified, but we can internally expand the block-feedback into nested feedbacks to make this easy. Lastly, the documentation needs to be reviewed.

[goerz]

@danielwe Can I assign you to this issue?

[danielwe]

That's OK. There will be about 4 weeks until I can start working on it.

(Note that on more careful reading, multiple channel feedback was always there in the original references. Due to the generality and redundancy of the abstract formalism it is actually admitted on two separate levels.)

We''l see what turns out to be the best approach: a single permutation step to bring all the lines into a contiguous corner block, or the more direct solution of building a non-diagonal adjacency matrix (essentially permuting the I in I - S_{ff} instead).

For visualization I think we should avoid expanding to nested single channel feedbacks. That does not result in the circuit diagram we want to see.