Provide another "NonSI" system in common?

[keilw]

As of now, some units like FRAMES_PER_SECOND are not part of UCUM (which has not changed in over 2 years) therefore it should not be in the UCUM system. There is no evidence, it's an official SI or ISO80k unit either, thus it can be seen as "domain specific". Either we should amend "uom-domain" with custom unit systems (like "health" does) or offer something in "common".