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   Model a spacecraft as an isothermal radiator.
   This model works only in Earth orbit.
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   Form:
   xDot = RHSIsothermal( x, t, d )
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   ------
   Inputs
   ------
   x        (1,1) Temperature
   t        (1,1) TIme
   d        (1,1) Data structure
                  .a1    (1,1) epsilon*a_r/cp*m
                  .a2    (1,1) alpha*a_s/cp*m
                  .a3    (1,1) 1/cp*m
                  .p     (1,1) Internal power
                  .jD0   (1,1) Julian date at t = 0
                  .rECI  (3,1) ECI Position vector
   jD       (1,n) Julian dates

   -------
   Outputs
   -------
   x        (1,1) Temperatures (deg-K)

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   References:	Agrawal, B. ,"Design of Geosynchronous Spacecraft,"
               Prentice-Hall, 1986, pp. 281-283
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