[IFC->XKT] Parse units from IFC

[elyrank]

if the IFC has a conversion measure unit - it is not updated in the xkt model i.e. : #43= IFCSIUNIT(*,.LENGTHUNIT.,$,.METRE.); #44= IFCDIMENSIONALEXPONENTS(1,0,0,0,0,0,0); #45= IFCMEASUREWITHUNIT(IFCRATIOMEASURE(0.3048),#43); #46= IFCCONVERSIONBASEDUNIT(#44,.LENGTHUNIT.,'FOOT',#45);

expected: viewer.scene.metrics.units === 'feet' actual: viewer.scene.metrics.units === 'meters'