Add suptitle to plot_bands

[lwasser]

Right now plot_bands returns only the axis object. this makes it difficult to add things like suptitles to a plot. We should consider one of the following

1. add suptitle as an argument or
2. have plot bands return both the figure and the ax list. that way you can manipulate fig if need be to add a suptitle.

ep.plot_bands(pre_arr,
figsize=(10,6))

this notebook -- 2020-03-02-landsat-multispectral-01-open-and-crop-all in the lessons repo has an example in a challenge at the bottom that i wish to add a suptitle to.