Changed Mass from class type to struct type.
Changed Mass from class type to struct type.

BoGrevyDynatest/UnitsNet,neutmute/UnitsNet,maherkassim/UnitsNet,anjdreas/UnitsNet,BrandonLWhite/UnitsNet,anjdreas/UnitsNet,maherkassim/UnitsNet,BrandonLWhite/UnitsNet,Tirael/UnitsNet