compute general relativistic correction to force on sun-centered
body.  based on eq. (6.34) of will, theory and experiment in
gravitational physics, cambridge univ. press, 1981.
(actually this equation is in isotropic rather than harmonic
coordinates, but these are the same to 1st post-newtonian order).

ncall=-2 setup once per iteration of a given step for partials
ncall=-1 setup once per iteration of a given step for motion
ncall= 0 setup once per step
ncall= k evaluate acceleration for equation k.gt.0

note on subscripts in this routine:
a,b,c (integers) match formula (6.34) or (6.78) in will
0  is sun
1-9 refer to planets (3 = embary, or earth if kgr(10).ge.0)
10  is moon if kgr(10).ge.0, else ignored
11  is integrated body (will duplicate if a planet)