Improves demo scripts

[neukym]

Fixes #24. Minor improvements for both the plotting demo, and the save/load demo, showing off more functionality than the originals.

@caiw - can you make sure you can run these yourself locally before accepting?

[caiw]

I don't seem to be able to add line-by-line comments on .ipynb files on Github. This file looks good to me except that the data file kymata_mirror_Q3_2023_expression_endtable.nkg is not included, so I can't actually run it.

Of course I have just fetched the file from Teams, so I can run the rest of the notebook, but I think we should discuss including/downloading data files (#1) before merging this in. (The referenced data file is, for example, 27 MB, though we have considered adding compression to the format: see #36).

[caiw]

This looks good, with the same caveat about relying on non-included data files.

I have a suggestion for this, but I can't do inline suggetions on notebook files (I think) so I will just make it and then pass the review back to @neukym.

[caiw]

Ok I can't request a review from @neukym but after you've informally reviewed my changes It's ready to merge.

[caiw]

@neukym, the new changes include changes from PR #41, so that should be reviewed first.