22372: EDF 2758 SMESH: Create/Manage groups on a mesh composed of nodes and balls

22372: EDF 2758 SMESH: Create/Manage groups on a mesh composed of nodes and balls

Performance optimization for groups on filter:

int SMESHGUI_Selection::numberOfNodes( int ind ) const
{
...
-        return aGroupObj->Size();
+        return aGroupObj->IsEmpty(); // aGroupObj->Size();