Refactor static variables in OpenSim::IO

[adamkewley]

Minor cleanup to #3943

• Moves static variables in IO into the cpp file as static-linkage variables
• Removes _locale static variable and instead just calls std::locale::classic() (no comments etc. were written to justify why it's worth caching - probably doesn't matter)
• Adds missing CHANGELOG.md entry for Improved String to Decimal parsing #3943

The reason I'm throwing this in separately is because it's much less tedious than slowly back-and-forthing on a separate PR. I'll merge this once it's through CI.

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This change is 

[nickbianco]

Reviewed 3 of 3 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @adamkewley)

[nickbianco]

Thanks @adamkewley!