Replace manual unit conversions with ud_convert() for consistency across models

[ayushman1210]

Description

This PR standardizes dimensional unit conversions by replacing manual arithmetic with the centralized helper function PEcAn.utils::ud_convert() where appropriate. This reduces maintenance burden, improves readability, and minimizes the risk of unit conversion errors.
The changes focus strictly on dimensional unit conversions (mass, time, energy, area) and intentionally exclude model-specific scaling, biological constants, and timestep-dependent logic, per project guidance.

Summary of Changes

• models/sipnet/R/model2netcdf.SIPNET.R
• Replaced manual mass unit conversions (gC → kgC) with ud_convert()
• Retained division by timestep.s for flux variables, since SIPNET outputs per-timestep accumulations (model-specific and required)
• Left biological constants and model-specific scalings unchanged (e.g., SLA, carbon fractions, cm→mm water conversions)
• models/dalec/R/model2netcdf.DALEC.R
• Replaced manual conversions (* 0.001, / 86400) with ud_convert()
• Removed redundant time division where ud_convert() already converts from daily to per-second units
• Ensured fluxes are consistently written as kgC m⁻² s⁻¹ and pools as kgC m⁻²
• models/gday/R/model2netcdf.GDAY.R
• Replaced manual Mg/ha ↔ SI conversions with ud_convert()
• Removed extra per-second scaling where ud_convert() already performs year → second conversion
• Kept pool variables as state quantities without time division
• models/sipnet/R/write_restart.SIPNET.R
• Replaced manual mass conversions with ud_convert()
• Removed redundant conversion factors while preserving model-specific logic
• models/stics/R/met2model.STICS.R
• Replaced precipitation conversion (kg m⁻² s⁻¹ → mm d⁻¹) with ud_convert()
• Left radiation and other physical constants unchanged where conversion is not purely dimensional

NOT CHANGED

• Biological / stoichiometric constants
• Model-specific timestep logic
• Physical constants mixed with unit conversions
• Plot- or model-convention scaling

Fixes - #3712

[infotroph]

Thanks for working on this! Can you describe how you tested these changes? I ask because I see a few issues here that should have produced obvious error messages in a test run. The issues themselves are minor and will be easy to fix, but since unit conversions can be surprisingly tricky it's important to know at review time that someone has actually run the code and looked at its output.

[ayushman1210]

hi @infotroph Thanks for the review and for pointing this out.
You’re right — I should have been clearer about testing. In this case, I did not run a full test or execute all code paths after making the changes. My checks were based on static reasoning and spot inspection, and a proper run would indeed have surfaced the issues you noticed.
I understand why this is especially important for unit conversions, and I agree that changes like this should always be validated by running the code and inspecting the output. I’ll fix the issues shortly and will make sure to:

• run the code locally,
• verify the relevant outputs,
• and explicitly describe the testing performed in the follow-up.

Thanks for the reminder !!

[ayushman1210]

Hi @infotroph
Just a gentle follow-up regarding the PR [#3719 ]
Whenever you get a chance, I’d really appreciate a review.
Thanks for your time!

[ayushman1210]

hey @infotroph sorry for the late response i have exams and college related work.
i have made changes as per your suggestion please review this pr
thanks !!

[infotroph]

The existing code here is using a timestep.s of 86400 (and not the 86400/out_day used in model2netcdf.SIPNET below), so I'm pretty sure these should all be converting from Mg/ha/day rather than Mg/ha/yr. Sorry I didn't catch this on my last review!

I recommend constructing a test to make sure the outputs are identical before and after these unit changes -- consider checking it with a subset of the example output in https://github.com/mdekauwe/GDAY/blob/master/example/outputs/D1GDAYDUKEAMB.csv