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

• Thelinearize command now preserves signal names for the linearized system, so that you can use signal names when forming andInterconnectedSystem.

• Interconnections ofLinearIOSystems are now treated asLinearICSystems (a subclass ofInterconnectedSystem andLinearIOSystem) so thatStateSpace functions will still work on them and they still keep track of the underlying interconnection structure.

• There is now aninterconnect function that allows the construction of anLinearICSystems or aInterconnectedSystem depending on whether the subsystems are all linear or not. This basically serves as a replacement for theconnect 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 sinceoutlist=(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:

• Therss anddrss functions now allow thestrictly_proper keyword, which sets the D matrix to zero. (I needed this to avoid occasional errors in unit tests for theinterconnect function.

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

• Added unit tests for all of the above.

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