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 #161.
• In response to #247.

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

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

Additional details and impacted files

```diff @@ 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](https://app.codecov.io/gh/HEPData/hepdata_lib/pull/251/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=HEPData) | Coverage Δ | | |---|---|---| | [unittests-3.10](https://app.codecov.io/gh/HEPData/hepdata_lib/pull/251/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=HEPData) | `87.99% <100.00%> (+0.28%)` | :arrow_up: | | [unittests-3.6](https://app.codecov.io/gh/HEPData/hepdata_lib/pull/251/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=HEPData) | `87.68% <100.00%> (+0.29%)` | :arrow_up: | | [unittests-3.7](https://app.codecov.io/gh/HEPData/hepdata_lib/pull/251/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=HEPData) | `87.68% <100.00%> (+0.29%)` | :arrow_up: | | [unittests-3.8](https://app.codecov.io/gh/HEPData/hepdata_lib/pull/251/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=HEPData) | `87.81% <100.00%> (+0.28%)` | :arrow_up: | | [unittests-3.9](https://app.codecov.io/gh/HEPData/hepdata_lib/pull/251/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=HEPData) | `87.81% <100.00%> (+0.28%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=HEPData#carryforward-flags-in-the-pull-request-comment) to find out more.

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