SYSML2_ — Quantity and unit for ratio and fraction

Explicit concepts for ratio, RatioValue and RatioUnit are missing from ISQ and SI, although they are highly relevant and often used.

Similarly fraction, FractionValue and FractionUnit would be useful.

Add these concepts as specializations of DimensionOneValue and DimensionOneUnit.

Also add units <'%'> percent, <ppm> 'parts per million', <pbm> 'parts per billion'. For parts per billion it is important to explicitly chose the US or UK billion (10^9 vs 10^12).