update psis to not add back the max and return the unnormalized resample ratios

[SteveBronder]

Fixes #3241 by just removing the line that adds back the max log likelihood ratio

Submission Checklist

• Run unit tests: ./runTests.py src/test/unit
• Run cpplint: make cpplint
• Declare copyright holder and open-source license: see below

Summary

Intended Effect

How to Verify

Should we have tests for this? This just seemed like a numeric bug inside of the function so I'm not sure if / how to test this

Side Effects

Documentation

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[avehtari]

Fixes #3241 by just removing the line that adds back the max log likelihood ratio

• it seems you also dropped the normalization, which is ok, if boost is accepting unnormalized weights
• it seems you also changed how the truncation is done, by using a one-liner instead of for loop, and as I'm not familiar with the syntax, I'm not able to verify that it does the same thing, but I assume you know it's the same