FEM: unit tests, fix denisty writing in existing unit tests

src/Mod/Fem/femtest/data/elmer/box_static_0_mm.sif

@@ -53,6 +53,7 @@ Body 1
 End
 
 Material 1
+  Density = Real 7.9e-06
   Poisson ratio = Real 0.3
   Youngs Modulus = Real 200000000.0
 End

src/Mod/Fem/femtest/data/elmer/ccxcantilever_faceload_0_mm.sif

@@ -53,6 +53,7 @@ Body 1
 End
 
 Material 1
+  Density = Real 7.9e-06
   Poisson ratio = Real 0.3
   Youngs Modulus = Real 210000000.0
 End

src/Mod/Fem/femtest/data/elmer/ccxcantilever_faceload_1_si.sif

@@ -54,6 +54,7 @@ Body 1
 End
 
 Material 1
+  Density = Real 7900.0
   Poisson ratio = Real 0.3
   Youngs Modulus = Real 210000000000.0
 End

src/Mod/Fem/femtest/data/elmer/ccxcantilever_nodeload_0_mm.sif

@@ -53,6 +53,7 @@ Body 1
 End
 
 Material 1
+  Density = Real 7.9e-06
   Poisson ratio = Real 0.3
   Youngs Modulus = Real 210000000.0
 End

src/Mod/Fem/femtest/data/elmer/ccxcantilever_prescribeddisplacement_0_mm.sif

@@ -53,6 +53,7 @@ Body 1
 End
 
 Material 1
+  Density = Real 7.9e-06
   Poisson ratio = Real 0.3
   Youngs Modulus = Real 210000000.0
 End