x     = output coordinates of center of mass of solar system
relative to sun determined from n-body tape
plus asteroid center-of-mass tape (if any)
output units are au and au/day
input quantities are
jd    = julian day number
fract = coordinate time fraction of day
nvel1 = 0 x(1-3) position output
nvel1 = 1 x(4-6) velocity output