This PR adds the function zpk, which creates a transfer function (as a NamedIOSystem) from a zero, pole, gain representation. This responds to issue #749.

Also added in code to make sure that signal names are copied correctly in various conversions, removing some comments entered in PR #804. In _convert_to_transfer_function this involved removing renaming from the low level function, as documented in the comment string.

Notes:

• Unlike in MATLAB and SciPy, which have zpk classes, here we just create a transfer function from the zpk data. That seems like the most common use case.

• In the matlab module there are also functions zpk2ss and zpk2tf that implement lower level behavior (eg, zpk2ss returns A, B, C, D). This is similar to MATLAB. if you want to get a StateSpace system from a zpk representation, use sys = ct.ss(ct.zpk(zeros, poles, gain)).

• Unit tests and updated documentation are also included.