Support overall figure title with subplots via gpos argument

[sadielbartholomew]

The rows and columns keyword arguments to gopen along with the gpos function allows one to create multiple subplot plots on one figure, but there is no built in-way to provide an overall title for the figure, to cover multiple plots.

We should support a simple, native way to add an overall title to the subplots. My suggestion is to allow this by providing a new keyword argument, overall_title or figure_title or similar, with a larger default font size that is configurable by another new keyword argument such as overalltitlefontsize.

Note at present, the only way to get an overall title is to defer to pyplot and use a suptitle (super title I believe this stands for), like so:

cfp.gopen(rows=3, columns=3)
plt.suptitle(<title>, fontsize=<fontsize>, <other args>)
...
cfp.gclose()

which is currently used for the second plot in the 12th documentation recipe as well as multiple new recipes in PR form, so as part of completing this we should update the recipes to use the new cf-plot built-in means to add the overall title.