Improve precision for large NDF #214
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For large ndf fraction `ndf / (ndf + x*x)` becomes close to 1 and taking logarithm loses precision. It's better to rewrite log(ndf / (ndf + x²)) = -log(1 + x²/ndf) = -log1p(x²/ndf)
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For large ndf fraction
ndf / (ndf + x*x)becomes close to 1 and taking logarithm loses precision. It's better to rewritelog(ndf / (ndf + x²)) = -log(1 + x²/ndf) = -log1p(x²/ndf)