Added invariant abbreviations for Microgram and Nanogram

Author: Paul Marvin

Src/UnitsNet/UnitSystem.cs

@@ -320,6 +320,8 @@ private void CreateCultureInvariants()
             MapUnitToAbbreviation(Unit.Decigram, "dg");
             MapUnitToAbbreviation(Unit.Centigram, "cg");
             MapUnitToAbbreviation(Unit.Milligram, "mg");
+            MapUnitToAbbreviation(Unit.Microgram, "µg");
+            MapUnitToAbbreviation(Unit.Nanogram, "ng");
 
             // Mass (imperial)
             MapUnitToAbbreviation(Unit.ShortTon, "short tn");