MAINT: Replace `plt.<cmd>` pattern with explicit fig/ax calls

[rossbar]

My understanding is that explicit figure/axis creation is the recommended pattern for pyplot on the basis of "explicit-over-implicit". Since one of the goals of numpy-tutorials is to highlight best-practices for scientific programming, I figured it'd be worthwhile to attempt to emphasize the OO pattern as opposed to the implicit fig/ax creation from plt.imshow, plt.plot, etc.

I don't have strong feelings though - I'm curious to what degree (if any) mpl folks would view this as an improvement @ksunden.