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   Used by fmins from SolarPanelThermal to compute panel temperatures.
   This returns a cost for the deviations of the front/back solar panel
   temperature equations from zero.
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   Form:
    [tF, tB] = SolarPanelThermal( d )
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   ------
   Inputs
   ------
   x               (2,1) [tF;tB] Front and back temperatures
   d               (1,1) Data structure
                         .alpha (1,1) Absorptivity
                         .epsF  (1,1) Front emissivity
                         .epsB  (1,1) Back emissivity
                         .etaP  (1,1) Fraction of flux to power
                         .k     (1,1) Front/back conductivity
                         .s     (1,1) Solar flux

   -------
   Outputs
   -------
   y               (1,1) Cost

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