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   Creates a cone with angular width 2*theta about the vector u.
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   Form:
   PlotCone( z, u, theta, fig, color )
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   Inputs
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   z             (3,1)     Location of cone point
   u             (3,1)     Center axis of the cone
   theta          (1)      Half-width of the cone [deg]
   fig            (1)      Figure handle (optional)
   color          (:)      Color of the cone (optional, default blue)

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   Outputs
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   none

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