Find better way to fix image cropping in docs.

Docs use Matplotlib's [`.. plot::` directive](http://matplotlib.org/devel/documenting_mpl.html?highlight=sphinx#module-matplotlib.sphinxext.plot_directive), which incorrectly crops off edges of images produced by plotting code (see Matplotlib issue [here](https://github.com/matplotlib/matplotlib/issues/4563), resulting in behavior like that below:


Need to find a better way to fix this other than sticking the following block of code before every `plt.show()` instance, as this obscures the `astroplan` code that the docs are meant to illustrate:

```
# Note that you don't need this code block to produce the plot.
# It reduces the plot size for the documentation.
ax = plt.gca()
box = ax.get_position()
ax.set_position([box.x0, box.y0, box.width * 0.75, box.height * 0.75])
```

Start by reading the documentation examples that use Matplotlib's `.. plot::` directive and reproduce the reported edge cropping. Compare the directive behavior with the per-example `plt.show()` workaround; done means documentation plots retain their image edges without adding that workaround to every example.

Written by the indexing model from the issue text.