[FEM] add current density constraint

- also add corresponding App::PropertyCurrentDensity

- also fix a typo in test_object.py

src/Mod/Fem/femobjects/constraint_currentdensity.py

@@ -0,0 +1,54 @@
+# ***************************************************************************
+# *   Copyright (c) 2023 Uwe Stöhr <uwestoehr@lyx.org>                      *
+# *                                                                         *
+# *   This file is part of the FreeCAD CAx development system.              *
+# *                                                                         *
+# *   This program is free software; you can redistribute it and/or modify  *
+# *   it under the terms of the GNU Lesser General Public License (LGPL)    *
+# *   as published by the Free Software Foundation; either version 2 of     *
+# *   the License, or (at your option) any later version.                   *
+# *   for detail see the LICENCE text file.                                 *
+# *                                                                         *
+# *   This program is distributed in the hope that it will be useful,       *
+# *   but WITHOUT ANY WARRANTY; without even the implied warranty of        *
+# *   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the         *
+# *   GNU Library General Public License for more details.                  *
+# *                                                                         *
+# *   You should have received a copy of the GNU Library General Public     *
+# *   License along with this program; if not, write to the Free Software   *
+# *   Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  *
+# *   USA                                                                   *
+# *                                                                         *
+# ***************************************************************************
+
+__title__ = "FreeCAD FEM constraint current density document object"
+__author__ = "Uwe Stöhr"
+__url__ = "https://www.freecadweb.org"
+
+## @package constraint_currentdensity
+#  \ingroup FEM
+#  \brief constraint body heat source object
+
+from . import base_fempythonobject
+
+
+class ConstraintCurrentDensity(base_fempythonobject.BaseFemPythonObject):
+
+    Type = "Fem::ConstraintCurrentDensity"
+
+    def __init__(self, obj):
+        super(ConstraintCurrentDensity, self).__init__(obj)
+        self.add_properties(obj)
+
+    def onDocumentRestored(self, obj):
+        self.add_properties(obj)
+
+    def add_properties(self, obj):
+        if not hasattr(obj, "CurrentDensity"):
+            obj.addProperty(
+                "App::PropertyCurrentDensity",
+                "CurrentDensity",
+                "Parameter",
+                "Current Density"
+            )
+            obj.CurrentDensity = 0.0