numpy.ndarray * control.TransferFunction gives array object #459
Description
TransferFunction doesn't have the __array_priority = 11 class attribute that StateSpace and FrequencyResponseData do. Is this on purpose? See below for results of array * tf, which results in an array (dtype object), and array * ss, which results in a SS object. I found this while investigating the behaviour of bdalg.connect in relation with #421
Should multiplication of scalar transfer functions by arrays be special-cased so that, e.g., with scalar tf k, k * eye(5) gives a 5x5 TF with identical entries on the diagonal? I think this sort of convention is used in literature, but it might be tricky to get something consistent here (e.g., should 1-input 1-output state-space systems do something analogous?; should one do general broadcasting?)
(control-dev) rory@rory-latitude:~/src/python-control$ ipython
Python 3.8.5 | packaged by conda-forge | (default, Jul 31 2020, 02:39:48)
Type 'copyright', 'credits' or 'license' for more information
IPython 7.17.0 -- An enhanced Interactive Python. Type '?' for help.
In [1]: !git describe
0.8.3-62-g6ede92e
In [2]: import control
In [3]: import numpy as np
In [4]: np.array([[1]]) * control.tf([1], [1,1])
Out[4]: array([[TransferFunction(array([1.]), array([1, 1]))]], dtype=object)
In [5]: np.array([[1]]) * control.ss([[-1]], [[1]], [[1]], [[0]])
Out[5]: StateSpace(array([[-1.]]), array([[1.]]), array([[1.]]), array([[0.]]))