Use viridis colormap

[tammojan]

This uses the viridis colormap, and includes support for many other colormaps. I'm not totally sure if just including the header of https://github.com/yuki-koyama/tinycolormap is allowed license-wise, perhaps @yuki-koyama can comment. I think setting up cmake to use submodules just for one file be a bit of an overkill.

[yuki-koyama]

Hi, tinycolormap is MIT-licensed. So, it is allowed to directly copy and include (and even modify if necessary) the header file into any other projects, as long as the original copyright and permission notices are included somewhere.

[tammojan]

Thanks @yuki-koyama! So only including the header file (and not deleting the copyright lines which are part of the header) is ok?

[yuki-koyama]

In my understanding, yes

[miek]

Thanks for the PR. I like the idea of this but I don't want to just swap out the current colormap with a new one.
I would be interested in a change that allowed different selectable colormaps though.