def resolve(cells)
      cells.each do |cell|
        cell_hash = cell.value
        if index = @unique_cells[cell_hash]
          cell.send :ssti=, index
        else
          cell.send :ssti=, @index
          @shared_xml_string << '<si>' << CellSerializer.run_xml_string(cell) << '</si>'
          @unique_cells[cell_hash] = @index
          @index += 1
        end
      end
    end
Interate over all of the cells in the array.
if our unique cells array does not contain a sharable cell,
add the cell to our unique cells array and set the ssti attribute on the index of this cell in the shared strings table
if a sharable cell already exists in our unique_cells array, set the ssti attribute of the cell and move on.
@return [Array] unique cells