Add mass to MDAnalysis base units

[sici17]

Fixes #3944

Changes made in this Pull Request:

• Added mass to the MDAnalysis.units base-units documentation table.
• Added "mass": "u" to MDANALYSIS_BASE_UNITS.
• Updated the corresponding base-units test.

I did not update package/CHANGELOG because this is a small documentation/base-units table fix.

The SI value used for u is 1.66053892103219 × 10^-27 kg, which is consistent with the CODATA 2010 constants already used in MDAnalysis.units.

Note that the User Guide currently lists a newer value for u (1.66053906660(50) × 10^-27 kg). This PR intentionally keeps the MDAnalysis.units API documentation consistent with the existing CODATA 2010 values used by the module, following the maintainer guidance in the issue.

LLM / AI generated code disclosure

LLMs or other AI-powered tools (beyond simple IDE use cases) were used in this contribution: yes

PR Checklist

• Issue raised/referenced?
• Tests updated/added?
• Documentation updated/added?
• package/CHANGELOG file updated?
• Is your name in package/AUTHORS? (If it is not, add it!)
• LLM/AI disclosure was updated.

Developers Certificate of Origin

I certify that I can submit this code contribution as described in the Developer Certificate of Origin, under the MDAnalysis LICENSE.

[read-the-docs-community]

Documentation build overview

📚 MDAnalysis | 🛠️ Build #33636596 | 📁 Comparing 6fe19af against latest (3d3a53e)

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4 files changed

± index.html
± documentation_pages/units.html
± _modules/MDAnalysis/units.html
± documentation_pages/analysis/wbridge_analysis.html