Make CT_LTI_System.tf callable

[adrianschlatter]

Currently, CT_LTI_System.tf returns the transfer-function representation as follows:

(matrix([[poly1d([ 5.48342176e+12])]], dtype=object),
poly1d([ 1.00000000e+00, 2.94538978e+05, 8.69700924e+12]))

As the transfer function is - as the name says - a function, it would be very convenient to make this callable. CT_LTI_System.tf could, e.g., return an object of class TransferFunction which has a nice repr and is additionally callable.