Don't unroll groups if property is WorkspaceProperty<WorkspaceGroup>

as this type indicates that the request is for the input to be the
whole group. Refs #6603

Code/Mantid/Framework/API/src/Algorithm.cpp

@@ -996,6 +996,9 @@ namespace Mantid
      *    - In this case, algorithms are processed in order
      *  - OR, only one input should be a group, the others being size of 1
      *
+     * If the property itself is a WorkspaceProperty<WorkspaceGroup> then
+     * this returns false
+     *
      * Returns true if processGroups() should be called.
      * It also sets up some other members.
      *
@@ -1016,7 +1019,8 @@ namespace Mantid
       m_groupWorkspaces.clear();
       for (size_t i=0; i<m_inputWorkspaceProps.size(); i++)
       {
-        Property * prop = dynamic_cast<Property *>(m_inputWorkspaceProps[i]);
+        auto * prop = dynamic_cast<Property *>(m_inputWorkspaceProps[i]);
+        auto * wsGroupProp = dynamic_cast<WorkspaceProperty<WorkspaceGroup> *>(prop);
         std::vector<Workspace_sptr> thisGroup;
 
         Workspace_sptr ws = m_inputWorkspaceProps[i]->getWorkspace();
@@ -1035,7 +1039,8 @@ namespace Mantid
         }
 
         // Found the group either directly or by name?
-        if (wsGroup)
+        // If the property is of type WorkspaceGroup then don't unroll
+        if (wsGroup && !wsGroupProp)
         {
           numGroups++;
           processGroups = true;