Added function that scans for known kerning group prefixes.
Tie the new prefixed group locating function into the converter.
Fix some failing tests.

I’m not sure why these were still broken.
Add write time kerning group down conversion to UFOWriter.

This allows the UFO 3 up converted kerning group names to be reset to
their original names at the time of writing in UFO <= 2.
Merge pull request #11 from robofab-developers/ufo3k-kerning-upconversion-additions

Incorporate additions to the kerning group conversion process.