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/FemMesh.cpp

@@ -2053,3 +2053,69 @@ Base::Quantity FemMesh::getVolume(void)const
 
 
 }
+
+int FemMesh::addGroup(const std::string TypeString, const std::string Name, const int theId)
+{
+    // define mapping between typestring and ElementType
+    // TODO: remove code doubling by providing mappings for all FemMesh functions
+    typedef std::map<std::string, SMDSAbs_ElementType> string_eltype_map;
+    string_eltype_map mapping;
+    mapping["All"] = SMDSAbs_All;
+    mapping["Node"] = SMDSAbs_Node;
+    mapping["Edge"] = SMDSAbs_Edge;
+    mapping["Face"] = SMDSAbs_Face;
+    mapping["Volume"] = SMDSAbs_Volume;
+    mapping["0DElement"] = SMDSAbs_0DElement;
+    mapping["Ball"] = SMDSAbs_Ball;
+
+    int aId = theId;
+
+    // check whether typestring is valid
+    bool typeStringValid = false;
+    for (string_eltype_map::const_iterator it = mapping.begin(); it != mapping.end(); ++it)
+    {
+        std::string key = it->first;
+        if (key == TypeString)
+            typeStringValid = true;
+    }
+    if (!typeStringValid)
+        throw std::runtime_error("AddGroup: Invalid type string! Allowed: All, Node, Edge, Face, Volume, 0DElement, Ball");
+    // add group to mesh
+    SMESH_Group* group = this->getSMesh()->AddGroup(mapping[TypeString], Name.c_str(), aId);
+    if (!group)
+        throw std::runtime_error("AddGroup: Failed to create new group.");
+    return aId;
+}
+
+void FemMesh::addGroupElements(const int GroupId, const std::set<int> ElementIds)
+{
+    SMESH_Group* group = this->getSMesh()->GetGroup(GroupId);
+    if (!group) {
+        throw std::runtime_error("AddGroupElements: No group for given id.");
+    }
+    SMESHDS_Group* groupDS = dynamic_cast<SMESHDS_Group*>(group->GetGroupDS());
+    // TODO: is this dynamic_cast OK?
+
+    // Traverse the full mesh and add elements to group if id is in set 'ids'
+    // and if group type is compatible with element
+    SMDSAbs_ElementType aElementType = groupDS->GetType();
+
+    SMDS_ElemIteratorPtr aElemIter = this->getSMesh()->GetMeshDS()->elementsIterator(aElementType);
+    while (aElemIter->more()) {
+        const SMDS_MeshElement* aElem = aElemIter->next();
+        std::set<int>::iterator it;
+        it = ElementIds.find(aElem->GetID());
+        if (it != ElementIds.end())
+        {
+            // the element was in the list
+            if (!groupDS->Contains(aElem)) // check whether element is already in group
+                groupDS->Add(aElem); // if not, add it
+        }
+    }
+}
+
+bool FemMesh::removeGroup(int GroupId)
+{
+    return this->getSMesh()->RemoveGroup(GroupId);
+}
+