Allow time_response_plot to plot a list of responses - #1254

Fixes #1171.

time_response_plot() failed when given a list of responses, as in the issue:

resp1 = ct.input_output_response(sys, timepts, np.sin(timepts))
resp2 = ct.input_output_response(sys, timepts, np.cos(timepts))
ct.time_response_plot([resp1, resp2])  # AttributeError: 'list' object has no attribute 'plot_inputs'

A list (or tuple) is now handed to TimeResponseList.plot(), which plots the responses one after the other on the same axes, so the result is the same as resp1.plot(); resp2.plot() (the plot the issue describes as the desired one) and as ct.TimeResponseList([resp1, resp2]).plot(). All keyword arguments, *fmt and a list of labels are passed through. I did not change the color scheme discussed at the end of the issue; the colors are the same as with TimeResponseList.plot() today.

Added test_time_response_plot_list to control/tests/timeplot_test.py (the example from the issue with plot_inputs None, True, 'overlay' and False, compared line by line with TimeResponseList.plot()). It fails on main and passes here. timeplot_test.py, timeresp_test.py, ctrlplot_test.py, kwargs_test.py and docstrings_test.py pass locally (542 passed, 51 skipped), and ruff check is clean.