Add scanning 3DXRD example notebooks

[jadball]

Adds some example notebooks for simple scanning 3DXRD data analysis.
The general plan is as follows:

1. These notebooks become the authoritative starting point for S3DXRD processing going forward
2. Update the Confluence documentation to point to the notebooks on GitHub
3. Make suggested changes/improvements by @jonwright
4. Move any scary maths out of the notebooks and into ImageD11 itself
5. Continue documentation of a few functions/figures that are still undocumented/uncommented
6. Do the same thing for box scanning