Rewrite plot functions to also work for updating a plot

The idea would be to have every cosseroots.plot function take three types of first arguments.

hp = cosseroots.plot.rod(rod, s, y, ___)
hp = cosseroots.plot.rod(ax, ___)
cosseroots.plot.rod(hp, ___)

where the first case would plot into a newplot, the second case in the given axes object, and the third one takes the group of graphics objects hp created and updates the data within to match the new rod data.