Cleaning Up Demos for Documentation

[CSSFrancis]

This pull request cleans up a lot of the broken parts of the demos, I should have maybe split it into different pieces, but now that is fine:

Cleaning up Structure:

• Applies a consistent structure to demos:

  • "# --> Heading"
  • "## --> Subheading"
  • "###--> So forth"

• Adds in tags so that some plots are used as nbextension-thumbnails

• Demo Name is listed at top of heading

Reducing Data Size:

• The data size was reduced for many of the dataset. Mostly through binning, cropping of datatype reduction
  • @magnunor I just used the cropped signal in the DPC example. There might need to be some additional work done there with regaurd to the text but for the most part that should work not
• All datasets are now under 100mb so they can be hosted on github (We could consider linking to larger datasets/ having a seperate set of lazy demos eventually but for now I think that this is good)

Fixing Demos:

• Additionally there were some errors when I ran some of the demos. For the most part (at least on my machine) They all run.

Support for Binder:

• This also adds in support for Binderizing the notebooks.

Demos in Documentation:

Most of these changes were motivated by having a set of example notebooks with can be used to create documentation.

If you want to look at how this works you can look at this PR https://github.com/pyxem/pyxem/pull/772

[pc494]

I'm going to merge this as it's larger than #75