m.e.ash   aug 1968   subroutine prtcrd (n-body format)
determination of perturbing planet coordinates
revised 5/26/66   to reduce storage requirements
y-vectors are calculated as they are needed

jd   = input julian day number
fract= input fraction of day
xpert(.,i)= output position,velocity,acceleration for
perturbing body i (vel,acc needed for rel)
rpert(i),rpert2(i),rpert3(i)= output distance, square, cube
for perturbing body i
xpert3(i,j) =xpert(i,j)/rpert3(j)   (i=1,3) (j=1,10)
ler     =  0 positions only needed for perturbing bodies
ler     =  1 velocities needed in relativity calculation
ler     =  2 velocities and accelerations needed in rel.cal.
merc =position of mercury from perturbing planet tape
body =position of venus through pluto from tape
moon =position of moon from tape
mdstau =moon distance unit on perturbing planet tape in au