create_backup Subroutine

    pure subroutine create_backup(particle, backup, istart, iend)
        class(particle_type), intent(in) :: particle  !! particle
        type(backup_type), intent(out) :: backup  !! backup
        integer, intent(in) :: istart, iend  !! start and end index

        allocate(backup%loc, source=particle%loc(:, istart:iend))
        allocate(backup%vel, source=particle%vel(:, istart:iend))
        allocate(backup%rho, source=particle%rho(istart:iend))
        allocate(backup%p, source=particle%p(istart:iend))

    end subroutine create_backup