This PR adds some new functionality for I/O systems that I used in a recent course on optimization-based control that I taught at Caltech:

• Modify thess(),rss(), anddrss() functions to returnLinearIOSystem objects (instead ofStateSpace objects). This makes it easier to create LTI state space systems that can be combined with other I/O systems without having to add a conversation step. SinceLinearIOSystem objects are alsoStateSpace objects, no functionality is lost. (This change is implemented through the introduction of a internalNamedIOSystem class, to avoid import cycles.)

• Added a new functioncreate_statefbk_iosystem() that creates an I/O system for implementing a linear state feedback controller of the form u = ud - Kp(x - xd). The function returns an I/O system that takes xd, ud, and x as inputs and generates u as an output. Theintegral_action keyword can be used to define a set of outputs y = C x for which integral feedback is also included: u = ud - Kp(x - xd) - Ki(C x - C xd).

• Thelqr anddlqr commands now accept anintegral_action keyword that allows outputs to be specified for implementing integral action. The resulting gain matrix has the form K = [Kp, Ki]. (This is useful for combining with theintegral_action functionality increate_statefbk_iosystem()).