Stumpff function C(z)
c = CL_stumpC(z)
Evaluates the Stumpff function C(z) according to the following equation :
input argument (1xN)
value of C(z) (1xN)
CNES - DCT/SB
1) Orbital Mechanics for engineering students, H D Curtis, Chapter 3 and Appendix D.4
2) Modern astrodynamics Fundamentals and perturbation methods, V Bond and M Allman, Chapter 6.2