Indexed circuit algebra operations

[goerz]

Since we now have an implementation of IndexedSum (and base classes for general indexed operations), it would make a lot of sense to extend the SLH algebra to allow for indexed Concatenation and SeriesProduct. The S-L-H operators in this case, and all algebraic operations, would involved indexed sums.

The reasoning behind this is that currently, the SLH algebra scales very badly with the number of nodes. At the same, time, networks with a "regular" structure, usually have a simple SLH-form when written with an indexed sum. Currently, we use QNET to evaluate the SLH for a small network, and try to (manually) deduce the general structure. It would be nice if this could be automatic, so that we can actually use QNET to handle arbitrarily large networks.