Update example formatting for better sphinx-gallery rendering

[speth]

Changes proposed in this pull request

• Add plots to several examples
• Add sphinx-gallery section headings / description blocks to several examples
• Adjust plotting in some examples to avoid re-use of existing figures (which doesn't work well with sphinx-gallery) and use plt.show() more consistently for when examples are run from the command line
• More extensive revisions to piston.py, mechanism_reduction.py, isentropic.py, and blowers_masel.py
• Fix some issues with output files and missing overwrite flags when saving 1D flame data

Checklist

• The pull request includes a clear description of this code change
• Commit messages have short titles and reference relevant issues
• Build passes (scons build & scons test) and unit tests address code coverage
• Style & formatting of contributed code follows contributing guidelines
• The pull request is ready for review

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 73.24%. Comparing base (fc2a16e) to head (2e6d7ee).
Report is 11 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1821      +/-   ##
==========================================
- Coverage   73.25%   73.24%   -0.01%     
==========================================
  Files         383      383              
  Lines       54624    54624              
  Branches     9103     9103              
==========================================
- Hits        40015    40012       -3     
- Misses      11603    11605       +2     
- Partials     3006     3007       +1     

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[bryanwweber]

Some small suggestions, thanks @speth!

[ischoegl]

Thanks, @speth!