Add LargeEnsemblePlotter

[pavlis]

Adds a new superclass to SeismicPlotter to handle plotting large ensembles. If one tries to plot more than a hundred or so signals at once you get a mysterious blank screen as all the plots are less than or near the size of one pixel. To avoid that this superclass automatically plots ensembles in multiple windows.

I also did some housecleaning on the SeismcPlotter class code.

We do not have pytest functions for graphics.py but I did use this version to (a) run our basic graphics tutorial and (b) for debugging a timing issue on real data on the IU cluster. It was very very helpful for the later. I don't know how or if one could use pytest for a graphical module like this.