2023 Update

Update at the start of 2023. Updated notebooks to the same coding standard as used for the next volume in the series (Stellar Structure and Evolution). This fixes issues with the notebooks due to updates especially to matplotlib. We also fixed a few issues with the earlier release notebooks that caused errors, or where we found better ways of making the plots.

Also made small changes to the readme file to align it with the new and upcoming volumes in the series. In particular, added download and update instructions using the git command line.

All users who downloaded the scripts before 2023 January 1 should update their copies following the instructions in the main readme.md file.