update colormaps in tutorials

[stevengj]

I would tend to update all of the tutorials that use bluered or dkbluered as the colormap to use RdBu instead, and anything that used hot to use inferno instead, for the reasons explained here: https://github.com/NanoComp/h5utils/blob/master/doc/h5topng-colors.md

cc @oskooi.

[stevengj]

To start with, it would be good to update https://github.com/NanoComp/meep/blob/master/doc/docs/Python_Tutorials/Basics.md