contour line scalar shading, add more perceptual colormaps

[keenancrane]

I swear I'm not trolling @nmwsharp. I just wanted a boring gray colormap and saw it wasn't available. I also added some recent perceptually uniform color maps that are standard in matplotlib, but not available in cmocean (magma, plasma, and inferno). This PR adds these color maps; nothing more, nothing less.

[nmwsharp]

Ha, my favorite PRs being with "I swear I'm not trolling"... But this is great, thanks!

I don't know if ti was intentional, but these colormap PRs were based on top of your old contour lines PR. However, I took that as a sign to finally get that one merged too.

I pushed commits with a few changes, mainly to the contour stuff: the contour lines are now treated as an optional alternate style for the existing "isoline" mod-stripe visualization. I did this to minimize the number of options/setters&getters -- they share a bunch of the same parameters anyway. I also standardized the width/frequency option to be called period, which is hopefully more informative.

[keenancrane]

Wow, I totally forgot about that PR (and apparently don't completely understand how to do PRs from a fork). But great! All sounds good to me. Thanks @nmwsharp.