r.king   june 1980   subroutine ertrel
evaluation of general relativity terms in right side of
equations of motion for the orbit of an earth satrellite.
called by ertorb and based on m.slade's code in monrel.

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