Gravity model allowing a point mass or harmonic models for a single body.
   This function has persistent memory and will use the last planet loaded. 
   The stored planet can be checked using 'planets'. The sun id is 0, the
   moon id is 10, and the major planets have id's 1 to 9.
   Designed for use with ode113. See also SailCombinedDemo, SPICombinedDemo.

   Since version 7.
            FOrbitSingle( 'add', planetName, d, nZ, nT )
            FOrbitSingle( 'add', id, mu )
   planet = FOrbitSingle( 'planets' )
     xDot = FOrbitSingle( t, x, rBodies, center, jD, aExt )

   t             (1,1)     Time
   x             (6,1)     State vector [r;v]
   rBodies         ()      Unused
   center          ()      Unused
   jD            (1,1)     Julian Date of epoch
   aExt          (3,1)     Additional external acceleration

   When data is entered on an 'add' call:
   planet (:)    Planet name string
   d      (:)    Gravity model structure, or mu for point mass model
                    s     (k,k)   S terms  s(n,m) where n and m are degree and order of the model
                    c     (k,k)   C terms  c(n,m)
                    j     (k)     m = 0 terms
                    mu    (1,1)   Spherical gravitational potential
                    a     (1,1)   Planet radius
   nZ    (1,1)   Highest zonal harmonic (m = 0)
   nT    (1,1)   Highest sectorial and tesseral harmonic

   d may also be a number

   xDot         (6,1)      State derivative
   planet        {1}       Cell array with planet name



Math: Linear/Mag
Orbit: OrbitMechanics/AGravityC
Sail: Coordinates/PlanetRot