This PR implements a couple of enhancements for the I/O systems package, mainly related to linear systems:

• The linearize command now preserves signal names for the linearized system, so that you can use signal names when forming and InterconnectedSystem.

• Interconnections of LinearIOSystems are now treated as LinearICSystems (a subclass of InterconnectedSystem and LinearIOSystem) so that StateSpace functions will still work on them and they still keep track of the underlying interconnection structure.

• There is now an interconnect function that allows the construction of an LinearICSystems or a InterconnectedSystem depending on whether the subsystems are all linear or not. This basically serves as a replacement for the connect function for LTI systems and allows named interconnection of linear or nonlinear systems.

In addition, I cleaned up the description for how to specify interconnections so that tuples are only used for the lowest level signal descriptions; everything else is a list. (Tuples will still work, but there are some ambiguities when you use tuples since outlist=(3, 2, 1) could be interpreted as a signal description for a single signal (subsystem 3, signal 2, with gain 1) or as a list of signals for SISO systems (the single outputs from subsystems 3, 2, and 1).

Additional small changes:

• The rss and drss functions now allow the strictly_proper keyword, which sets the D matrix to zero. (I needed this to avoid occasional errors in unit tests for the interconnect function.

• Added some configuration variables that allow you to change the way that states in interconnected systems are named (default was sys.state but is sys_state for 0.9+) and the way that duplicate systems are named (default was copy of sys but is sys$copy for 0.9+).

• Added unit tests for all of the above.

• Updated examples to match the newly documented conventions for signal list specification.