imshow is deprecated, replace with matplotlib about crepe HOT 4 CLOSED

using matplotlib for pixel-perfect drawing was a little pain, dealing with dpi and figimage() because with pyplot.imshow it's even more difficult to deal with the margins.

I don't see using deprecated scipy.misc.imsave as an immediate problem, as long as we freeze the dependency. If it bugs you, I'd prefer using imageio as suggested by the doc, rather than pyplot.

matplotlib dependency is also not a huge problem as most of our audience would have it installed anyway, but it bugs me a little bit in the pure software engineering perspective, because the only thing that we're using is the inferno colormap. Those are linear colormaps and can be reimplemented with < 10 lines of code.

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I think we should definitely avoid using deprecated modules in a new release, especially since it's been deprecated since 1.0.0.

I don't see the point of importing both matplotlib AND imageio, we should just use one. I'm not sure I follow what you mean by "pitch perfect drawing"? Anyway, I'm happy for us to switch to imageio if you prefer... we just need to decide.

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I meant to type pixel-perfect.. but anyway, let's do imageio for saving images, and we can save grayscale images instead of inferno for the time being.

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Replaced scipy.misc.imsave with imageio.imwrite in 592e493

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