InitializeSailGravity:

% Initialize the sail simulation gravity model.
 Clears the necessary functions and sets up data consistently.
 Uses FOrbitSingle or FOrbitGeneral, along with the appropriate 
 ephemeris function.

--------------------------------------------------------------------------
   Forms:
   d = InitializeSailGravity( d, bodyNames, models, nZ, nT, ids )
   d = InitializeSailGravity( d, 'earth', mu )
--------------------------------------------------------------------------

   ------
   Inputs
   ------
   d         (:)    Initial simulation struct
   bodyNames {n}    Body names, with the first being the center
   models    {n}    Cell array of model names, or gravitational constants, 
                      or harmonic model structs
   nZ       (1,n)   Number of zonal harmonics
   nT       (1,n)   Number of tesseral harmonics
   ids      (p,1)   Additional planets IDs to add as point sources

   -------
   Outputs
   -------
   d         (:)    Simulation struct with gravity fields added

--------------------------------------------------------------------------

Children:

Common: Database/Constant
Common: General/AssignFHandle
Sail: OrbitDynamics/FOrbitGeneral
Sail: OrbitDynamics/FOrbitSingle