SYSML2 — Extend ISQ with missing quantity and unit types for US Customary Units

The US Customary Units as specified in NIST SP811 Annex B include a number of units for which the corresponding quantity is not defined in ISO/IEC 80000, the International System of Quantities (ISQ). These units are currently commented out in Quantities and Units Domain Library USCustomaryUnits. Examples are:

//attribute <'Btu_IT/ft³'> 'British thermal unit (IT) per cubic foot' : EnergyDensityUnit = Btu_IT/ft^3;
//attribute <'gal/(hp⋅h)'> 'gallon (US) per horsepower hour' : EnergySpecificVolumeUnit = gal/(hp*h);
//attribute <'mi/gal'> 'mile per gallon (US)' : FuelEconomyUnit = mi/gal;
//attribute <'lb/(hp⋅h)'> 'pound per horsepower hour' : FuelConsumptionUnit = lb/(hp*h);

An additional extension package for ISQ is needed that declares the attribute definitions for the missing quantity and measurement unit types, e.g.:

• EnergyDensityValue and EnergyDensityUnit
• EnergySpecificVolumeValue and EnergySpecificVolumeUnit
• FuelEconomyValue and FuelEconomyUnit
• FuelConsumptionValue and FuelConsumptionUnit

It should be considered whether or not to include the new library package inside ISQ, as formally these quantity and unit types are not part of ISO/IEC 800000.