Subroutine PLCMC - J.F.Chandler - 1977 December 1
Compute corrections to coordinates in 'XP' for planet center of
mass instead of system center -- type of coordinate indicated
by iabs(NV)
NV=0 - position, NV=1 - velocity, NV=2 - acceleration
If NV=0 or 1, then XP is a 6-vector of position and velocity,
but if NV=2, then XP is 3-vector of acceleration
Controls set up in PLCMS (called from CMPAR3)

JCT(55) is a packed bits indicator for the planet center of mass
computations
JCT(55)=0  no center of mass corrections
1 bit = 1: get satellite coordinates from interpolation if possible
2 bit = 1: get satellite coordinates from elliptic if necessary
4 bit = 1: use only satellites that are on s-body tape
8 bit = 1: for elliptic method, obtain elements from s-body tape
(if possible).  Warning: this option should normally
be set because the initial conditions in /EMPCND/ are
likely to have been overwritten from tape anyway and
may not match the epoch in CON1(1).
16 bit = 1: allow calculation of barycenter of planet+observed
satellite instead of planet
32 bit = 1: use precessing elliptic orbit instead of stationary