Switch LTI class and subclasses to use ninputs, noutputs, nstates

[murrayrm]

This PR changes the LTI class and the StateSpace class (and all derived classes) to use ninputs, noutputs, and nstates as the attributes for storing these properties instead of inputs, outputs, and states. The former names are consistent with what is used in the InputOutputSystem class.

Following the suggestion of @bnavigator in issue #451, getter and setter functions are used for backward compatibility, but with a PendingDeprecationWarning.

Note that this PR will generate some small conflicts with #514. I'll rebase based on which one we merge first. [done]

[coveralls]

Coverage decreased (-0.06%) to 87.553% when pulling eb401a9 on murrayrm:input_output_to_ninput_noutput into 1502d38 on python-control:master.

[sawyerbfuller]

Awesome! Looks good to me.

[sawyerbfuller]

One more thing I noticed: there are a few mentions of sys.inputs/outputs in docs in xferfunc.py that should probably be fixed.

[murrayrm]

Rebased against master (and apparently fixed up the xferfcn.py docstring, since I can't find sys.inputs in that file anymore).

[sawyerbfuller]

May also need to check for self.inputs ?

[sawyerbfuller]

One more comment: was it a deliberate choice to leave init keywords as inputs/outputs rather than ninputs/noutputs, eg in lti.py:

def init(self, inputs=1, outputs=1, dt=None):

[murrayrm]

was it a deliberate choice to leave init keywords as inputs/outputs rather than ninputs/noutputs,

This matches the syntax we use in the InputOutputSystem class constructor. In that setting (and perhaps later here as well), inputs and outputs can be an integer or a list of strings (which get used for signal names).

[sawyerbfuller]

That makes sense. Thanks!

[bnavigator]

This raises an Exception of the class PendingDeprecationWarning and never returns self.inputs.

Python 3.8.6 (default, Nov 09 2020, 12:09:06) [GCC] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import control
>>> rsys = control.rss(3,3,3)
>>> ninputs = rsys.inputs
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/ben/src/python-control/control/lti.py", line 64, in inputs
    raise PendingDeprecationWarning(
PendingDeprecationWarning: The LTI `inputs` attribute will be deprecated in a future release.  Use `ninputs` instead.
>>> ninputs
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'ninputs' is not defined

Suggested change

	raise PendingDeprecationWarning(
	"The LTI `inputs` attribute will be deprecated in a future "
	"release. Use `ninputs` instead.")
	return self.ninputs
	warn("The LTI `inputs` attribute will be deprecated in a future "
	"release. Use `ninputs` instead.",
	PendingDeprecationWarning, stacklevel=2)
	return self.ninputs

Additionally, either a warning filter needs to be set in order to make the warning visible to end users, or a different category than *DeprecationWarning needs to be used.

filterwarnings('module',
               message=".*LTI.*attribute.*deprecated",
               category=PendingDeprecationWarning)

Python 3.8.6 (default, Nov 09 2020, 12:09:06) [GCC] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import control; rsys = control.rss(3,3,3); ninputs = rsys.inputs
<stdin>:1: PendingDeprecationWarning: The LTI `inputs` attribute will be deprecated in a future release.  Use `ninputs` instead.
>>> ninputs
3

• Please add a check using pytest.warns().

[murrayrm]

I've fixed these up and decide to use DeprecationWarning for now (keep things internal to developers) but in a future release (0.9.x) we should change to FutureWarning, which will make it show up for users as well.

[bnavigator]

Still using raise instead of warn. Now we are at:

This raises an Exception of the class DeprecationWarning and never returns self.inputs.

Python 3.8.6 (default, Nov 09 2020, 12:09:06) [GCC] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import control; rsys = control.rss(3,3,3); ninputs = rsys.inputs
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/ben/src/python-control/control/lti.py", line 64, in inputs
    raise DeprecationWarning(
DeprecationWarning: The LTI `inputs` attribute will be deprecated in a future release.  Use `ninputs` instead.
>>> ninputs
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'ninputs' is not defined
>>> 

[bnavigator]

The assert .. == part is never evaluated after sys.ninputs throws the exception.

[murrayrm]

Sorry, was working on multiple changes/branches at once and pushed the wrong version. New one was what was intended.