[Base] add electrical conductivity

- this is necessary to perform electrodynamics simulations. Since the typical values are in the range of 1e6 S/m, we need the new units kS and MS

- since this is for simulations, also add electrical conductivity to the FEM material cards

src/Base/Unit.h

@@ -121,6 +121,7 @@ public:
     static Unit ElectricalInductance;
     static Unit ElectricalConductance;
     static Unit ElectricalResistance;
+    static Unit ElectricalConductivity;
     static Unit AmountOfSubstance;
     static Unit LuminousIntensity;