FEM: fix unicode conversion error (sifio.py)

The sifio module could only serialize the str type. This commits
adds support for unicode strings. Some properties of FreeCAD
objects like the equation variable name use unicode strings.

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

@@ -394,12 +394,14 @@ class _Writer(object):
             return _TYPE_REAL
         if issubclass(dataType, str):
             return _TYPE_STRING
+        if issubclass(dataType, unicode):
+            return _TYPE_STRING
         raise ValueError("Unsupported data type: %s" % dataType)
 
     def _preprocess(self, data, dataType):
         if issubclass(dataType, Section):
             return str(self._idMgr.getId(data))
-        if issubclass(dataType, str):
+        if issubclass(dataType, str) or issubclass(dataType, unicode):
             return '"%s"' % data
         return str(data)