This PR contains a refactoring of the code that is used to implement the I/O and LTI systems classes, building on some of the changes started in PR #710 and adding new functionality (and consistency). There are quite a few changes, but everything is either new functionality or backward compatible and so existing code does not need to change.

Summary of changes:

• Changed the LTI class to use poles() and zeros() for retrieving poles and zeros, with pole() and zero() generating a PendingDeprecationWarning (which is ignored by default in Python). (The MATLAB compatibility module still uses pole() and zero().)
• The TimeResponseData and FrequencyResponseData objects now implement a to_pandas() method that creates a simple pandas dataframe.
• The FrequencyResponseData class is now used as the output for frequency response produced by freqresp() and a new function frequency_response has been defined, to be consistent with the input_output_response function. A FrequencyResponseData object can be assigned to a tuple to provide magnitude, phase, and frequency arrays, mirroring TimeResponseData functionality from PR Time response data class #649.
• The LTI class is now a subclass of NamedIOSystem (formerly _NamedIOSystem), which allows state space and transfer functions systems to have system and signal names. In addition, the separate _NamedIOStateSystem class was eliminated in favor of using nstates to distinguish a system with no state space functionality (nstates == None, used for TransferFunction and FrequencyResponseData systems) from a system defined in state space (nstates >= 0, used for InputOutputSystem and StateSpace systems).
• The __repr__ method for NamedIOSystem has been updated to implement the changes in draft PR Change InputOutputSystem repr to show type, name, inputs, and outputs #699 and the __str__ method has been updated for linear I/O system to combine the information for an I/O system and a state space system. In particular, I/O systems are now displayed as SystemClass:sysname:[input_labels]->[output_labels]>.
• Common functionality for the LTI and InputOutputSystem classes is now pulled up into the NamedIOSystem class (and namedio.py) and a common parsing function _process_namedio_keywords() is used for consistent behavior across system creation functions.
• The drss, rss, ss2tf, tf2ss, tf2io, and ss2io functions now all accept system and signal name arguments (via _process_namedio_keywords().
• • The rss function can now accept a dt argument and can create either continuous-time or discrete-time random systems. The drss function just calls rss() with dt=True.
• The ss function can now accept function names as arguments, in which case it creates a NonlinearIOSystem (I'm not sure how useful this is, but ss is a sort of wrapper function that calls the appropriate class constructor, so it was easy enough to implement.)
• The interconnect function now has a warn_duplicate flag that can be used to turn on/off warning messages about duplicate objects. The default value is None, in which case warnings are suppressed for duplicated systems with generic names (of the form sys[n]. (The let's you do things like H = G * G without getting warning messages for state space systems without specified names.)
• The SystemTrajectory class for flat systems now has a response() method that generates a TimeDataResponse object.
• Added some missing type conversions between systems by using the __array_priority__ class variable to allow the right operand to override the left operand when needed. This mainly affect the FrequencyResponseData class.
• Updated unit tests, docstrings, user documentation, and examples to match new functionality.

The following diagram (from the user documentation) illustrates the relationship between the various classes: