Continue NURBS gird manager with entity and iterator class

This week I continue my work with NURBS gird manager. The following work have been done

• Created the NURBSGridLeafIterator class
   template<int codim, typename NURBSGridView, typename NURBSEntity>
  class NURBSGridLeafIterato
  which allows iteration over all entities of a NURBSGridView
  
• Modified the NURBSPatch class and NURBSGrid
  to fit the newly-add features
  
• Created a NURBSGridLeafIndexSet
  template<class GridImp>
  class NURBSGridLeafIndexSet
  to return the index of a given entity

• Added test case for NURBS grid

The implementation details are hidden as it is analogous to what I did for BSpline grid.

The next step is to test whether what we had is enough for a VTKwriter. Now in our test code, we iterate over all elements of a BSpline/NURBS grid we created, and writes the resulting geometry manually to a VTK file. DUNE provides a VTKwriter which is able to write the elements of a gridView automatically to a VTK file. But we don't know yet what interfaces are required to make the VTKwriter work expected. So we will try to debug into and see what new classes and functions are needed.