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   Model a spacecraft as an isothermal radiator.
   This is useful for preliminary estimates of radiator area.
   The spaceccraft illuminated area is aS with radiators of area aR.
   You enter the orbit and the center (any of the nine planets or the sun)
   plus the orbital elements and spacecraft properties. The function will
   compute the temperature as a function of time.
   This function includes solar, albedo and radiation.
   In a heliocentric orbit planetary encounters are ignored. In planetary
   orbit albedo, planetary radiation and eclipses are modeled.

   For a demo of a geosynchronous spacecraft type Isothermal.

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   Form:
   t = Isothermal( el, center, d, jD )
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   Inputs
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   el       (1,6) Orbital elements [a i W w e M]
   center   (1,:) Name of center (major planets and sun)
   d        (1,1) Data structure
                  .aR    (1,1) Radiator area (m^2)
                  .aS    (1,1) Spacecraft illuminated area (m^2)
                  .alpha (1,1) Spacecraft absorption (0-1)
                  .epsR  (1,1) Radiator emissivity (0-1)
                  .t0    (1,1) Initial Temperature (deg-K)
                  .cP    (1,1) Average specific heat of the spacecraft (J/kg deg-K)
                  .m     (1,1) Spacecraft mass  (kg)
   jD       (1,n) Julian dates

   -------
   Outputs
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   t        (1,n) Temperatures (deg-K)

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