FitPair:

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   Given xF, find yF that best fits xF given the pair (x,y)
   xF may repeat in x. Does a linear interpolation for yF.

   For a demo type FitPair.

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   Form:
   yF = FitPair( x, y, xF )
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   Inputs
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   x               (1,:) x values
   y               (1,:) y values
   xF              (1,1) Value to be fit

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   Outputs
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   yF              (1,n) Corresponding y values

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