GeezerVison

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:

• The NamedIOSystem class has been renamed InputOutputSystem and serves as the parent class for all input/output system classes.
• The LinearIOSystems class has been merged into the StateSpace class.
• Instances of NonlinearIOSystem can now be created using the nlsys() factory function (instead of using the class name, so more like ss and tf).
• 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 to input_output_response).
  • The impulse_response function only works for LTI systems.
• Extraneous arguments for simulation functions have been removed (see add/cleanup documentation on simulation functions #905):
  • The impulse_response function no longer accepts the X0 keyword.
  • The initial_response function no longer accepts the input keyword.
  • The step_response function still accepts the X0 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 (via InputOutputSystem.__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. (The pole() and zero() functions are still available in the MATLAB compatibility module, control.matlab.)
• The connect function generates a DeprecationWarning (use interconnect instead).
• Configuration defaults in the namedio module are now in the iosys 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 the LinearIOSystem class with the StateSpace class.
• Replace pole() and zero() with poles() and zeros().

Additional recommended changes:

• Replace calls to ss2io or tf2io with calls to ss or tf, including moving signal and system naming parameters into the ss or tf call.
• Replace calls to StateSpace, TransferFunction, and NonlinearIOSystem with calls to the equivalent factory functions: ss(), tf(), nlsys().