differences on `fprime`. `fhess_p`
must compute the hessian times an arbitrary vector. If it is not
given, finite-differences on `fprime` are used to compute
it.

Newton-CG methods are also called truncated Newton methods. This
function differs from scipy.optimize.fmin_tnc because

1. scipy.optimize.fmin_ncg is written purely in Python using NumPy
    and scipy while scipy.optimize.fmin_tnc calls a C function.
2. scipy.optimize.fmin_ncg is only for unconstrained minimization
    while scipy.optimize.fmin_tnc is for unconstrained minimization
    or box constrained minimization. (Box constraints give
    lower and upper bounds for each variable separately.)

Parameters `c1` and `c2` must satisfy ``0 < c1 < c2 < 1``.

References
----------
Wright & Nocedal, 'Numerical Optimization', 1999, p. 140.

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