Clarify behaviour if zero uncertainties

[GraemeWatt]

• Add a paragraph to the documentation to explain how to encode missing bins.
• Add an option zero_uncertainties_warning to the Variable class with default value True.
• Add a test for a data point with zero uncertainties. This should have been added as part of PR Do not write errors if all errors are zero for bin #161.
• In response to Allow dummy uncertainties for variable member with dummy (string) value #247.

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📚 Documentation preview 📚: https://hepdata-lib--251.org.readthedocs.build/en/251/

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (2287c18) 87.70% compared to head (765680e) 87.99%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #251      +/-   ##
==========================================
+ Coverage   87.70%   87.99%   +0.28%     
==========================================
  Files           5        5              
  Lines        1082     1083       +1     
  Branches      231      231              
==========================================
+ Hits          949      953       +4     
+ Misses         93       91       -2     
+ Partials       40       39       -1     

Flag	Coverage Δ
unittests-3.10	87.99% <100.00%> (+0.28%)	⬆️
unittests-3.6	87.68% <100.00%> (+0.29%)	⬆️
unittests-3.7	87.68% <100.00%> (+0.29%)	⬆️
unittests-3.8	87.81% <100.00%> (+0.28%)	⬆️
unittests-3.9	87.81% <100.00%> (+0.28%)	⬆️

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[clelange]

Thanks, looks good to me! I'm just wondering whether at some point hepdata_lib should actually support this, so we can keep the issue open for now.