Get g, the covariant toroidal magnetic field component, on the psi mesh
integer, intent(in) :: ns
real(i2mex_r8), intent(in) :: psi(ns)
real(i2mex_r8), intent(out) :: g(ns)
integer, intent(out) :: ier
    Get d g /d psi on psi mesh
    Get d^2 g /d psi^2 on psi mesh
integer, intent(in) :: ns
real(i2mex_r8), intent(in) :: psi(ns)
real(i2mex_r8), intent(out) :: gpp(ns)
integer, intent(out) :: ier