Added Metric Ton NonSI unit accepted in the SI standard.

Refactored the run-time suffix handling by defining QuantityUnit type and BaseUnitSI type instead of an awkward baseFactor + baseUnit duo. The thing is, the SI standard is separating the quantity suffix from the base unit SI suffix. "The SI is founded on seven SI base units for seven base quantities". Those two roles are mapped into the QuantityUnit and BaseUnitSI strings and used accordingly.
Added Day Non SI unit supported by the SI standard.

Place also PowerRatio and Ratio dimensionless unit in the Non SI namespace.
Added Mass constructors tests.

Unstable commit because a bug in the Kilogram to gram conversion has been discovered.