While use the iosys module, I noticed that the following construct did something unexpected:

lateral = ct.NonlinearIOSystem(vehicle_update, vehicle_output,
    states=2, name='lateral', inputs=('phi'), outputs=('y', 'theta')
)

This is supposed to be a single input, multi-output system, but I got a system with 3 (?) inputs. Turns out that the string 'phi' was being interpreted as an array of length three since I didn't write it explicitly as a tuple, ('phi',) and hence it was an array of length 3.

This PR checks to see if inputs, outputs, or states are a single string object, in which case it assumes you want are specifying a single signal with the string as the name.

I also fixed a small bug where the prefix for setting the default name (if no name string is given for a single) was being ignored.