m.e.ash   april 1965   subroutine elipt
optimized 1977 j.f.chandler
code moved to jnitl/jlipt  1977 dec  j.f.chandler
subroutine elipt determines position and velocity in the encke
elliptic orbit and the partial derivatives of position and
velocity in this orbit with respect to the orbital elements
nn=0 position and velocity determined
nn=-1,1 position,velocity,and partial derivatives determined