LinearNetwork should initialize parent class

[arvoelke]

Currently this is not done as a hack in order to prevent num and den from being frozen. But this means that:

nengolib.signal.s._params

will raise an exception, because:

c:\users\aaron\ctn\nengo\nengo\params.pyc in _params(self)
    435     @property
    436     def _params(self):
--> 437         return itervalues(self._paramdict)
    438 
    439     def __eq__(self, other):

AttributeError: 'LinearSystem' object has no attribute '_paramdict'

Thanks @psipeter.