def add_group(name)
      return nil unless @document
      group = Xcodeproj::Workspace::GroupReference.new(name)
      elem = @document.root.add_element(group.to_node)
      yield group, elem if block_given?
      group
    end
def add_group(name)
      return nil unless @document
      group = Xcodeproj::Workspace::GroupReference.new(name)
      elem = @document.root.add_element(group.to_node)
      yield group, elem if block_given?
      group
    end
-------------------------------------------------------------------------
 Adds a new group container to the workspace
 workspace.

 @param  [String] name The name of the group

 @yield [Xcodeproj::Workspace::GroupReference, REXML::Element]
        Yields the GroupReference and underlying XML element for mutation

 @return [Xcodeproj::Workspace::GroupReference] The added group reference