Replace gridPlot with cowplot:plot_grid

[Jason Vander Heiden]

It's worth considering replacing the builtin gridPlot function with cowplot::plot_grid because it has a lot of additional useful features.

I think it's also worth looking at cowplot::theme_cowplot. I wouldn't want to replace baseTheme with it (because I like the look of baseTheme better), but there's some more fine grain control in the cowplot theme function we might want to replicate.