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   Calculates the time from pulse start to the controid.
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   Form:
   [tc, u] = PCentr( taur, tauf, pulsewidth )
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   ------
   Inputs
   ------
   taur             (1,1) Rise time constant (sec)
   tauf             (1,1) Fall time constant (sec)
   pulsewidth       (1,1) Pulsewidth (sec)

   -------
   Outputs
   -------
   tc               (1,1) Time from start of pulse to centroid (sec)
   u                (1,1) Effective pulsewidth (sec)

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