Completion + tidying

[pp-mo]

This branch was intended for a few structural changes, plus response to a set of offline comments.

Unfortunately the structural changes have involved a lot of moving content between notebooks, which is never easy to follow via git, even with normal plain-text files.

I still have some outstanding points, but for now maybe we should take an overview of the outcome alreday achieved here. Sorry for the difficulty and non-iterative change, but I don't think there was any decent alternative.

A rough summary of the change content here ...

• various improvments to style + consistency of wording and punctuation
• rationalised heading levels for sections + subsections
• added a final "Next notebook" para to all the core notebooks
• standardised on "small data" loading
  • -- but for now, this means the C48 data, not the reduced-dimensions data which @BHFock has produced, which is currently incompatible (moree work to do)
• moved initial mesh investigation exercises from end of Sec#01 to Sec#02
• moved quick plotting from end of Sec#02 to Sec#03
• split out the "back into a cube" part from end of Sec#04 into a separate "bonus" notebook ("MeshCube_Extraction")
• replaced the "click to reveal" sections with open code cells (at least mostly)
• tidied up the overview notebook "Mesh_Tutorial_Intro" quite a lot.
  • upgraded the 'jargon'/glossary section with lots of links
  • added section on GeoVista planar projected plots -- i.e. 2D plotting

Things known still wanted (to do in subsequent PRs ?)

• make everything work with even smaller data (as produced by @BHFock)
• adopt additional change suggestions to Sec#01 notebook, banked in this commit on BHFock/main

[pp-mo]

We believe this is the best we have so far, at any rate better than what went before -- except for the outstanding work tasks noted in the header comment. Just merging for the latest + best of everything.