Fix ACCESS-ESM1-6 nbp mapping formula

[rbeucher]

Summary

• update ACCESS-ESM1-6 land nbp mapping to use: -1 * (fld_s03i100 * 12 / 44.01) / fld_s03i395
• align model_variables and model_variable_units with the new formula

Closes #153.

Related context: previously closed #115.

Forum discussion: https://forum.access-hive.org.au/t/early-access-esm1-6-ilamb-evaluation-pipeline-using-access-moppy/6378/25

[rbeucher]

Linking this PR to previously closed issue #115 for historical context.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.1%. Comparing base (ec0c181) to head (cbe9cdb).

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #502   +/-   ##
=====================================
  Coverage   78.1%   78.1%           
=====================================
  Files         33      33           
  Lines       6626    6626           
  Branches    1260    1260           
=====================================
  Hits        5173    5173           
  Misses      1180    1180           
  Partials     273     273           

Flag	Coverage Δ
unit	78.1% <ø> (ø)

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