[FEM] Elmer: add permeability of vacuum

src/Mod/Fem/femsolver/elmer/writer.py

@@ -194,6 +194,7 @@ class Writer(object):
         self.constsdef = {
             "Gravity": constants.gravity(),
             "StefanBoltzmann": constants.stefan_boltzmann(),
+            "PermeabilityOfVacuum": constants.vacuum_permeability(),
             "PermittivityOfVacuum": constants.vacuum_permittivity(),
             "BoltzmannConstant": constants.boltzmann_constant(),
         }

src/Mod/Fem/femtools/constants.py

@@ -42,6 +42,11 @@ def stefan_boltzmann():
     return "5.67037e-8 W/(m^2*K^4)"
 
 
+def vacuum_permeability():
+    # https://en.wikipedia.org/wiki/Vacuum_permeability
+    return "1.256637e-6 N/A^2"
+
+
 def vacuum_permittivity():
     # https://forum.freecadweb.org/viewtopic.php?f=18&p=400959#p400959
     # https://en.wikipedia.org/wiki/Permittivity#Vacuum_permittivity