fix: use dataReloadRatio as multiplier instead of its complement in SWD v4

[alexzurbonsen]

Summary

In the original formula of the SWD v4 model the data cache ratio is used and the emissions per byte are multiplied with (1 - data cache ratio). The CO2js implementation uses the data reload ratio = (1 - data cache ratio). But this is not reflected in the computation, which still multiplies by (1 - data reload ratio).

This PR fixes that.

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• Adds a new carbon estimation model to CO2.js
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• Fixes a bug with an existing model implementation
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• Improves developer experience for contributors
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[fershad]

This addresses #248 in a way - however it keeps the "data reload ratio" value rather than aligning with the "data cache ratio" used in the Sustainable Web Design methodology.

There's a question here about whether we stick to the SWD methodology, or do we stray from it slightly in implementation. For reference, currently, the docs refer to dataReloadRatio and explain it in the right way (data downloaded by return visitors).

@mrchrisadams do you have any thoughts about this?

[alexzurbonsen]

@fershad Interesting, I wasn't aware of that.

For me personally I don't mind if the variables in this implementation do not correspond to the ones from the SWD docs, as long as the underlying formula and the results are the same.

But if you are considering a bigger make over, I would also rename firstVisitPercentage and returnVisitPercentage to their original names newVisitorRatio and returnVisitorRatio, since they are no percentages (which could also take values like 20 or 100) but ratios (between 0 and 1). I am even wondering why both of these variables can be set independently of each other, since they are complements of each other, i.e. newVisitorRatio = 1 - returnVisitorRatio.

[fershad]

Updated test results and merged in the latest changes from main.

Expecting to merge this PR back into main later this week & release a patch.

[alexzurbonsen]

Thanks a lot @fershad

[fershad]

One thing to do before submitting is to add some notes into the code (comments) that explain what dataReloadRatio is and how it compares to Data Cache Ratio which is found in the SWDMv4 documentation.