Motivated by PR #246, which implemented indexing for transfer functions, it would be nice to be able to use Python slice notation for MIMO system objects. So if you had a MIMO transfer function G, you could say things like G[:,1:3] to get a subset of the transfer functions.

A full implementation should allow slice functionality to be used in simulation and plotting functions, block diagram functions (issue #20), etc.

Will use this issue to keep track of some of the possibilities and see if there is a good way forward.