Use SI.Mass instead of specifying 'unit'

This is more elegant, and also similar to how it's done in Modelica.Media.Interfaces.PartialMedium.MassFlowRate:
  type MassFlowRate = SI.MassFlowRate(quantity = "MassFlowRate." + mediumName, min = -1.0e5, max = 1.e5)

Modelica/Media/package.mo

@@ -2316,7 +2316,7 @@ is given to compare the approximation.
         fixed=true,
         stateSelect=StateSelect.prefer),
       X(start={0.8,0.2}));
-    Real m1(unit = "kg", quantity = "Mass." +  Medium1.mediumName, start=1.0);
+    SI.Mass m1(quantity = "Mass." +  Medium1.mediumName, start=1.0);
     SI.InternalEnergy U1;
     Medium1.SpecificHeatCapacity cp1=Medium1.specificHeatCapacityCp(medium1.state);
     Medium1.DynamicViscosity eta1=Medium1.dynamicViscosity(medium1.state);
@@ -2332,7 +2332,7 @@ is given to compare the approximation.
         fixed=true,
         stateSelect=StateSelect.prefer),
       X(start={0.1,0.1,0.1,0.2,0.2,0.3}));
-    Real m2(unit = "kg", quantity = "Mass." +  Medium2.mediumName, start=1.0);
+    SI.Mass m2(quantity = "Mass." +  Medium2.mediumName, start=1.0);
     SI.InternalEnergy U2;
     Medium2.SpecificHeatCapacity cp2=Medium2.specificHeatCapacityCp(medium2.state);
     Medium2.DynamicViscosity eta2=Medium2.dynamicViscosity(medium2.state);