General gravity model allowing point mass and harmonic models.
   This function has persistent memory and should be cleared between 
   implementations. The stored planets can be checked using 'planets'.
   When loading a simple list of mu's for major planets (including the sun), 
   numeric ids and vector inputs can be used. See PlanetPosJPL.
   Designed for use with ode113. See also AsteroidOrbitDemo.

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

   t             (1,1)     Time
   x             (6,1)     State vector [r;v]
   rBodies       (3,n)     Positions of bodies in same frame as x
   center        (1,1)     ID of center in stored list
   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



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