6.4 ACCW: add to data on file

      subroutine accw(a,len,ifil,name,ioffs,factor,new)

This routine is functionally equivalent to the code

      if (.not.new) then
        call lesw(buf,len,ifil,name,ioffs)
      else
        call fzero (buf,len)
      end if
      call daxpy(len,factor,a,1,buf,1)
      call sreibw(buf,len,ifil,name,ioffs)

and thus achieves a daxpy operation for a vector on disk rather than in memory. It should be used wherever possible in preference to separate calls to lesw and sreibw, since its implementation on parallel platforms is significantly more efficient.