Interp1D:

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   Performs a linear interpolation:
   y = (y2-y1)*(x-x1)/(x2-x1) + y1

   Since version 1.
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   Form:
   y = Interp1D( y1, y2, x1, x2, x )
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   ------
   Inputs
   ------
   y1                 First value
   y2                 Second value
   x1                 First value
   x2                 Second value
   x                  Input

   -------
   Outputs
   -------
   y                  Interpolated value

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