Added invariant abbreviations for Microgram and Nanogram
Added invariant abbreviations for Microgram and Nanogram

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