OptTestFunParabola:

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   A simple parabola, the function f = x^2

   Since version 7.
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   Form:
   f = OptTestFunParabola( x, d )
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   Inputs
   ------
   x              (1,1)  Value of x
   d              (1,1)  Structure to pass to fun (not used)

   -------
   Outputs
   -------
   f              (1,1)  Cost

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