Always create Axes objects in examples

[dstansby]

Our rules of thumb for gallery examples exist to "minimize verbosity and maintain consistency with the other gallery examples:". In some places consistency and verbosity are a trade off, including which flavour of the Matplotlib API we are calling.

Broadly the two options are:

1. Use pyplot (e.g. plt.xlabel) where possible, and do not create Axes objects

2. Use Axes all the time, even when it would be possible to use pyplot

3. is a bit less verbose than 2., but this comes at the cost of having two ways of doing stuff in our gallery (e.g. plt.xlabel() vs. ax.set_xlabel(). In my opinion consistency is more important than minimizing verbosity here, so I am proposing that we always create an Axes object.

I've converted one gallery example as an example of what this change would look like. Thoughts?

[ayshih]

I usually favor avoiding the pyplot interface, but I'll point out that the non-pyplot alternative to a plain plt.colorbar() call is notably more clunky.

[Cadair]

I am generally in favour of this, it works nicer with WCSAxes and our custom plotting wrappers.

[wtbarnes]

I don't really have an opinion here as long as we are consistent across the whole gallery.

[dstansby]

Grand - I'll go through and change everything then, and then take this back out of draft for some proper reviews on the actual implementation.

[wtbarnes]

I initially started replacing plt.subplots with fig = plt.figure(); ax = fig.add_subplot() but ran out of steam. I don't particularly like the inconsistency in the ways we create the figure, axes objects, but I do not think it is worth the added verbosity.