nyquist_response ignores indent_direction unless poles have exactly zero real part #1194

As noted in #1191, the current code for nyquist_response ignores the indent_direction parameter unless there are poles with exactly zero real part. An alternative behavior would be something like this:

• If indent_direction is None, then for any pole within indent_radius of the imaginary axis, use the real part of the pole location to determine which way to indent. If the pole is exactly on the imaginary axis, then indent to the right (current default).

• If indent_direction is either 'left' or 'right' and the pole is within indent_radius of the imaginary axis, then indent in the specified direction.

This seems more intuitive and allows you to use the indent direction when poles are near the imaginary axis but not exactly on it.