[metapost] Units unaccuracy with numbersystem set to double

[Akira Kakuto]

> Maybe this problem has been solved in this last version.

MetaPost 1.900 shows

*show cm;
>> 28.346450000000001

Note that MetaPost does not calculate it from an exact formula.
It simply prints the value defined in plain.mp: cm=28.34645;

Best,
Akira