NewtonSolver:

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   Solves for the zeros of a set of n equations.
   The solver is based on the Taylor series

   f(x) = f(x0) + f'(x0)*(x - x0)
   
   Both f(x0) and f'(x0) are required. If f' is not supplied, it will
   compute it numerically using a first order Jacobian.

   For a demo type

   NewtonSolver

   for the equations

   0 = x(1) + x(2)^2 - 2
   0 = x(1)^2 - 1

   Since version 7.
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   Form:
   [x, k, err] = NewtonSolver( fun, x, opt, d )
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   ------
   Inputs
   ------
   fun (1,:) Function containing the equations to be solved
   x   (:,1) Initial guess
   opt (1,1) Data structure with optional parameters
             .nMax   (1,1) Maximum number of iterations (Default 100)
             .tol    (1,1) Tolerance on weighted error (Default 1e-10)
             .w      (:,1) Weight on each equation (Default ones(:,1))
             .funJac (1,:) Jacobian function (Default internal numerical)

   -------
   Outputs
   -------
   x   (:,1) Final values
   k   (1,1) Number of iterations
   err (:,1) Errors

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