Orbit propagation using "secular J2" model
kep_t2 = CL_ex_secularJ2(t1,kep_t1,t2 [,er,mu,j2])
Propagates orbital elements considering the mean secular effects due to J2 only.
Notes:
- There can be 1 or N initial times, and 1 or N final times.
- This function works for elliptical orbits only.
The type of orbital elements is the following:

Initial time [days] (1x1 or 1xN)
Orbital elements at time t1 (6x1 or 6xN).
Final time [days] (1xN or 1x1).
(optional) Equatorial radius [m] (default is %CL_eqRad)
(optional) Gravitational constant [m^3/s^2] (default value is %CL_mu)
(optional) (Unnormalized) zonal coefficient (second zonal harmonic) (default is %CL_j1jn(2))
Orbital elements propagated to time t2. (6xN)
CNES - DCT/SB