signal.cont2discrete not handling lti instances as documented

[pgorczak]

The docs for cont2discrete say that it takes an instance of the lti class as a possible argument. I could not make this work with a system created via TransferFunction.

Example:

import scipy.signal as sig
g = sig.TransferFunction([1], [1, 1])
print(isinstance(g, sig.lti))
gd = sig.cont2discrete(g, dt=0.01)

This snippet prints True and then raises TypeError: object of type 'TransferFunctionContinuous' has no len().

My current workaround is using the to_discrete member function instead:

gd = g.to_discrete(dt=0.01)

The exception occurs at the first line in cont2discrete, which checks for len(system) == 1, seemingly to determine if it's an instance of lti. Maybe this could simply be replaced with an isinstance call? Looks like dt should also be passed on in the line after.