write_connectivity Function

   function write_connectivity(self, nc, connectivity, offset, cell_type, face, faceoffset) result(error)
   !< Write mesh connectivity.
   !<
   !< **Must** be used when unstructured grid is used, it saves the connectivity of the unstructured gird.
   !< @note The vector **connect** must follow the VTK-XML standard. It is passed as *assumed-shape array*
   !< because its dimensions is related to the mesh dimensions in a complex way. Its dimensions can be calculated by the following
   !< equation: \(dc = \sum\limits_{i = 1}^{NC} {nvertex_i }\).
   !< Note that this equation is different from the legacy one. The XML connectivity convention is quite different from the
   !< legacy standard.
   !< As an example suppose we have a mesh composed by 2 cells, one hexahedron (8 vertices) and one pyramid with
   !< square basis (5 vertices) and suppose that the basis of pyramid is constitute by a face of the hexahedron and so the two cells
   !< share 4 vertices. The above equation gives \(dc=8+5=13\). The connectivity vector for this mesh can be:
   !<
   !<##### first cell
   !<+ connect(1)  = 0 identification flag of \(1^\circ\) vertex of first cell
   !<+ connect(2)  = 1 identification flag of \(2^\circ\) vertex of first cell
   !<+ connect(3)  = 2 identification flag of \(3^\circ\) vertex of first cell
   !<+ connect(4)  = 3 identification flag of \(4^\circ\) vertex of first cell
   !<+ connect(5)  = 4 identification flag of \(5^\circ\) vertex of first cell
   !<+ connect(6)  = 5 identification flag of \(6^\circ\) vertex of first cell
   !<+ connect(7)  = 6 identification flag of \(7^\circ\) vertex of first cell
   !<+ connect(8)  = 7 identification flag of \(8^\circ\) vertex of first cell
   !<
   !<##### second cell
   !<+ connect(9 ) = 0 identification flag of \(1^\circ\) vertex of second cell
   !<+ connect(10) = 1 identification flag of \(2^\circ\) vertex of second cell
   !<+ connect(11) = 2 identification flag of \(3^\circ\) vertex of second cell
   !<+ connect(12) = 3 identification flag of \(4^\circ\) vertex of second cell
   !<+ connect(13) = 8 identification flag of \(5^\circ\) vertex of second cell
   !<
   !< Therefore this connectivity vector convention is more simple than the legacy convention, now we must create also the
   !< *offset* vector that contains the data now missing in the *connect* vector. The offset
   !< vector for this mesh can be:
   !<
   !<##### first cell
   !<+ offset(1) = 8  => summ of nodes of \(1^\circ\) cell
   !<
   !<##### second cell
   !<+ offset(2) = 13 => summ of nodes of \(1^\circ\) and \(2^\circ\) cells
   !<
   !< The value of every cell-offset can be calculated by the following equation: \(offset_c=\sum\limits_{i=1}^{c}{nvertex_i}\)
   !< where \(offset_c\) is the value of \(c^{th}\) cell and \(nvertex_i\) is the number of vertices of \(i^{th}\) cell.
   !< The function VTK_CON_XML does not calculate the connectivity and offset vectors: it writes the connectivity and offset
   !< vectors conforming the VTK-XML standard, but does not calculate them.
   !< The vector variable *cell\_type* must conform the VTK-XML standard (see the file VTK-Standard at the
   !< Kitware homepage) that is the same of the legacy standard. It contains the
   !< *type* of each cells. For the above example this vector is:
   !<
   !<##### first cell
   !<+ cell\_type(1) = 12 hexahedron type of first cell
   !<
   !<##### second cell
   !<+ cell\_type(2) = 14 pyramid type of second cell
   class(xml_writer_abstract), intent(inout) :: self             !< Writer.
   integer(I4P),               intent(in)    :: nc               !< Number of cells.
   integer(I4P),               intent(in)    :: connectivity(1:) !< Mesh connectivity.
   integer(I4P),               intent(in)    :: offset(1:)       !< Cell offset.
   integer(I4P),   optional,   intent(in)    :: face(1:)         !< face composing the polyhedra.
   integer(I4P),   optional,   intent(in)    :: faceoffset(1:)   !< face offset.
   integer(I1P),               intent(in)    :: cell_type(1:)    !< VTK cell type.
   integer(I4P)                              :: error            !< Error status.

   call self%write_start_tag(name='Cells')
   error = self%write_dataarray(data_name='connectivity', x=connectivity)
   error = self%write_dataarray(data_name='offsets', x=offset)
   error = self%write_dataarray(data_name='types', x=cell_type)
   call self%write_end_tag(name='Cells')

   !< Add faces and faceoffsets to the cell block for polyhedra. If the cell is not a polyhedron, its offset must be set to -1.
   if(present(face).and. present(faceoffset)) then
        error = self%write_dataarray(data_name='faces', x=face)
        error = self%write_dataarray(data_name='faceoffsets', x=faceoffset)
   endif
   endfunction write_connectivity