Take ICU Complexity into consideration and reconsider the term "Compound"

[keilw]

I stumbled over MeasureUnit.Complexity in the latest version of ICU4J.

It makes a clear distinction between COMPOUND (being something like "meter-per-second") and MIXED (like "hour-and-minute")
As we luckily have not yet propagated any of that into the API, let's clean up the terminology after another formal standard like Unicode/ICU which we also support via systems-unicode picked those terms.