write_fielddata_tag Function

   function write_fielddata_tag(self, action) result(error)
   !< Write `<FieldData>`/`</FieldData>` start/end tag.
   class(xml_writer_abstract), intent(inout) :: self      !< Writer.
   character(*),               intent(in)    :: action    !< Action: **open** or **close** tag.
   integer(I4P)                              :: error     !< Error status.
   type(string)                              :: action_   !< Action string.

   action_ = trim(adjustl(action)) ; action_ = action_%upper()
   select case(action_%chars())
   case('OPEN')
      call self%write_start_tag(name='FieldData')
   case('CLOSE')
      call self%write_end_tag(name='FieldData')
   endselect
   error = self%error
   endfunction write_fielddata_tag