Use the gain crossover 'wc' as a first guess for solving the phase crossover frequency

There were a plotting error with the bode() function and ZiergerNicols function in utils.py because the phase crossover frequency 'w_180' returned by margins() was negative (and can't plot a negative number on a log axis).
There are multiple solutions for w_180 for each revolution (360n) and depending on the initial guess this could cause the solution to be out by a phase 360n. There is only one solution for wc and w_180 should be close to wc, therefore wc should be a good first estimate for w_180