added new pnsr formula

[magic-peach]

created a new pnsr formula
fixes #11

[Gooichand]

⚠️ Misleading Title + Merge Conflict

Thanks for working on Issue #11! However:

Problem 1: Title says "added new pnsr formula" but you just refactored the existing PSNR (changed 20*log10 to 10*log10 which is mathematically equivalent). No new formula was added.

Problem 2: This conflicts with PR #46 - both modify comparison.py. PR #46 should merge first (it's better documented), then you'll need to rebase.

Questions:

1. What was the bug in Issue PSNR formula in ComparisonWidget is mathematically incorrect #11? The math looks correct in both versions.
2. Did you mean to add a different PSNR variant (PSNR-HVS, PSNR-HMA)?

Please clarify and update the title to "Refactor PSNR calculation" if it's just code cleanup. 👍