Use double math until CSV output time in Interop 2022 metrics

[foolip]

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[foolip]

This will undo the 0.1% changes to the output caused by #88, but it's not a plain revert since lots of comments are updated.

@jgraham @gsnedders would you like to review? I personally prefer having 0-1000 integers at every level, from here and in the frontend. It increases the accumulated error, but is easy to understand.

Aside: I think that if we add support in wpt.fyi for showing percentage scores at test and directory levels, we wouldn't want to put doubles on the wire because of the size, it's more likely we'd work with 0-100 or 0-1000 integers there.

[foolip]

I have moved this to web-platform-tests/results-analysis#67 ahead of archiving this repo.