This PR restructures the input/output systems classes as described in issue#904 and summarized in the following diagram:

In addition, a number of deprecated functions were removed and various other changes related to I/O systems were made.

Summary of major changes:

• TheNamedIOSystem class has been renamedInputOutputSystem and serves as the parent class for all input/output system classes.
• TheLinearIOSystems class has been merged into theStateSpace class.
• Instances ofNonlinearIOSystem can now be created using thenlsys() factory function (instead of using the class name, so more likess andtf).
• All operations that used to work on a subset of I/O system classes should now work on all compatible I/O system classes:
  • Block diagram functions (series,parallel,feedback,append,neg) now work on all I/O system classes, including nonlinear systems.
  • Simulation functions (initial_response,step_response,forced_response) will now work for nonlinear functions (via an internal call toinput_output_response).
  • Theimpulse_response function only works forLTI systems.
• Extraneous arguments for simulation functions have been removed (seeadd/cleanup documentation on simulation functions #905):
  • Theimpulse_response function no longer accepts theX0 keyword.
  • Theinitial_response function no longer accepts theinput keyword.
  • Thestep_response function still accepts theX0 keyword (to allow for non-zero equilibrium points for nonlinear systems).
• The__str__() method for I/O system classes now includes the name of the system and signals (viaInputOutputSystem.__str__()).
• Redundant functions have been marked as deprecated:ss2io,tf2io.
• Deprecated functions and properties have been removed:pole,zero,LTI.{inputs,outputs, pole, zero},timebaseEqual. (Thepole() andzero() functions are still available in the MATLAB compatibility module,control.matlab.)
• Theconnect function generates aDeprecationWarning (useinterconnect instead).
• Configuration defaults in thenamedio module are now in theiosys module.
• Updated user documentation, docstrings, examples, and unit tests.

What you will need to change to run version 0.10.0 (this should go in the release notes):

• Replace any use of theLinearIOSystem class with theStateSpace class.
• Replacepole() andzero() withpoles() andzeros().

Additional recommended changes:

• Replace calls toss2io ortf2io with calls toss ortf, including moving signal and system naming parameters into thess ortf call.
• Replace calls toStateSpace,TransferFunction, andNonlinearIOSystem with calls to the equivalent factory functions:ss(),tf(),nlsys().