FIX: LAMMPSDUMP Convert forces from kcal to kJ units

[Rupesh-Singh-Karki]

Fixes #5115

Changes made in this Pull Request:

• Add units attribute to DumpReader class specifying LAMMPS "real" units (time: fs, length: Angstrom, velocity: Angstrom/fs, force: kcal/(mol*Angstrom))
• Implement force unit conversion in _read_next_timestep() method to convert forces from native kcal/(molAngstrom) to MDAnalysis base units kJ/(molAngstrom)
• Add unit conversion calls for positions, velocities, and forces when convert_units=True (default behavior)
• Update docstring to document the automatic force unit conversion behavior
• Add comprehensive tests for force unit conversion functionality:
  • Test units attribute definition
  • Test conversion factor calculation (4.184)
  • Test actual force conversion using existing test files
  • Test conversion consistency across all trajectory frames
• Ensure backward compatibility with convert_units=False option

This fix resolves the inconsistency where LAMMPSDUMP forces remained in kcal/(molAngstrom) while other trajectory formats (GROMACS TRR, AMBER NetCDF) properly converted forces to kJ/(molAngstrom). Now all trajectory formats use consistent force units for downstream analysis.

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!)

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.

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📚 Documentation preview 📚: https://mdanalysis--5117.org.readthedocs.build/en/5117/

[codecov[bot]]

Codecov Report

:x: Patch coverage is 46.93878% with 26 lines in your changes missing coverage. Please review. :white_check_mark: Project coverage is 85.82%. Comparing base (519ac56) to head (518bca9). :warning: Report is 27 commits behind head on develop.

Files with missing lines	Patch %	Lines
package/MDAnalysis/coordinates/LAMMPS.py	46.93%	18 Missing and 8 partials :warning:

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Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #5117      +/-   ##
===========================================
- Coverage    93.88%   85.82%   -8.07%     
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  Files          180      180              
  Lines        22422    22496      +74     
  Branches      3186     3202      +16     
===========================================
- Hits         21052    19308    -1744     
- Misses         902     2722    +1820     
+ Partials       468      466       -2     

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[orbeckst]

@hmacdope @jaclark5 could you review this PR? I think you both have some LAMMPS experience. Or can you think of someone else?

[orbeckst]

@hmacdope I have tentatively assigned you as PR shepherd. If you don't have the bandwidth, please find someone else or un-assign yourself and let me know. Thanks.