Adding `ScatterPlot` and `PlotGrid` to `plotting`.

Adding classes to `plotting` module that handle most of the heavy
lifting for making grids of scatter plots.

Also adding `saturation_plot` function that uses these classes to create
a msbayes summary statistics saturation plot.
Updating `dmc_saturation_plot.py` script.

Now using `saturation_plot` method from `plotting` module to handle
all of the plotting.