Allow dgsh-tee to both read and write from/to multiple channels

[dspinellis]

For N inputs and M outputs:

• if N >= M and N mod M = 0 and N / M = k: Copy input from N0...N(k -1) to M0, Nk...N(2*k - 1) to M1, ...
• if N < M and M mod N = 0 and M / N = k: Copy input from N0 to M0...M(k - 1), N1 to Mk .. M(2*k - 1), ...

Note that reading from one or writing to one is just a special case of the above.