Refactor: Make td_write more compact

Make td_write more compact by adding an array of file names, td_file_name, allowing me to replace the same if-block for numerous files with:

   do ifile = 1, OUT_MAX
      ! Exceptions that are handled later
      if (any([OUT_MULTIPOLES, OUT_LASER, OUT_FTCHD] == ifile)) cycle

        if (writ%out(ifile)%write) then
          call write_iter_init(writ%out(ifile)%handle, &
            first, &
            units_from_atomic(units_out%time, dt),  &
            trim(io_workpath("td.general/"//trim(td_file_name(ifile)), namespace)))
        end if

      enddo 

Documentation is also added for this subroutine, plus the classes in the module.