Compute general relativistic correction to force on sun-centered body
for n-body integration.  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).

RVEC  - VECTORS TO BODY 1 FROM BODY 2
RAB   - DISTANCES
V     - VELOCITIES (IN SSBC FRAME)
A     - INDEX OF INTEGRATED BODY IN LIST NPLBDY
A < Nsun   EVALUATE ACCELERATION FOR BODY A
A = Nsun   SETUP ONCE PER ITERATION
SUMR  - RETURNED VECTOR GR ACCELERATION ON BODY A

note on subscripts in this routine:
a,b,c (integers) match formula (6.34) or (6.78) in will
a is integrated body
last body in list is sun