Quadplot

Added a quadplot class to handle 4 panel plots (bode plots left, bode plots right).
Also modified the residuals method to take advantage of the quadplot class.
Added example_quad.py
Modified example_plot.py to account for some minor changes in plot.residuals()
Added a test in test_plot.py

Breaking down merge request !120 (closed) to smaller pieces.

Partially addresses issue #57