[FEM] fix handling of vacuum permittivity

- fix message that permittivity object was not handled
- set an expression, otherwise the user gets only "0.000" for the default of 8.8e-12
- use as unit the common "F/m" (this is also more user-friendly than "s^4*A^2 / (m^3*kg)")
- fix a ToDo

src/Base/UnitsSchemaInternal.cpp

@@ -435,6 +435,10 @@ QString UnitsSchemaInternal::schemaTranslate(const Quantity &quant, double &fact
             factor = 1e6;
         }
     }
+    else if (unit == Unit::VacuumPermittivity) {
+        unitString = QString::fromLatin1("F/m");
+        factor = 1;
+    }
     else if (unit == Unit::Frequency) {
         if (UnitValue < 1e3) {
             unitString = QString::fromLatin1("Hz");