FEM: add methods to to edit mesh groups:

- add addGroup, addGroupElements, removeGroup to C++ mesh class
- expose the methods to Python
- add a unit test class
- update test commands file
- add test to fem test app module

src/Mod/Fem/App/FemMeshPy.xml

@@ -158,6 +158,37 @@
                 <UserDocu>Return a tuple of ElementIDs to a given group ID</UserDocu>
             </Documentation>
         </Methode>
+        <Methode Name="addGroup" Const="true">
+            <Documentation>
+                <UserDocu>Add a group to mesh with specific name and type
+                    addGroup(name, typestring, [id])
+                    name: string
+                    typestring: \"All\", \"Node\", \"Edge\", \"Face\", \"Volume\", \"0DElement\", \"Ball\"
+                    id: int
+                    Optional id is used to force specific id for group, but does
+                    not work, yet.
+                </UserDocu>
+            </Documentation>
+        </Methode>
+        <Methode Name="addGroupElements" Const="true">
+            <Documentation>
+                <UserDocu>Add a tuple of ElementIDs to a given group ID
+                    addGroupElements(groupid, list_of_elements)
+                    groupid: int
+                    list_of_elements: list of int
+                    Notice that the elements have to be in the mesh.
+                </UserDocu>
+            </Documentation>
+        </Methode>
+        <Methode Name="removeGroup" Const="true">
+            <Documentation>
+                <UserDocu>Remove a group with a given group ID
+                    removeGroup(groupid)
+                    groupid: int
+                    Returns boolean.
+                </UserDocu>
+            </Documentation>
+        </Methode>
         <Methode Name="getElementType" Const="true">
             <Documentation>
                 <UserDocu>Return the element type of a given ID</UserDocu>