regrid Subroutine

  subroutine regrid (nold, x, y, nnew, xnew)
    use splines, only: new_spline, splint, delete_spline, spline
    implicit none
    integer, intent (in) :: nold
    real, dimension (-nold:nold), intent (in) :: x
    real, dimension (-nold:nold), intent (in out) :: y
    integer, intent (in) :: nnew
    real, dimension (-nnew:nnew), intent (in) :: xnew
    type (spline) :: spl
    integer :: i

    spl = new_spline (x(-nold:nold), y(-nold:nold), tension = regrid_tension)

    do i = -nnew, nnew
       y(i) = splint(xnew(i), spl)
    end do

    call delete_spline (spl)
  end subroutine regrid