"value_type" aka. Unit beliefs, is named a bit ambiguously

[cppcooper]

https://github.com/DFHack/df-structures/blob/a04727cc4ec350399ce4d2ded4425f33c50cea50/df.units.xml#L146-L151

It would be nice if this was named similar to the wiki article, if only for the sake of searchability.

edit: as per discord discussion, the game seems to refer to this system as a value system. @Ozzatron arrived on personality_value_type as an alternative to simply value_type