PLTRP - Dec. 1975 - R. Goldstein
Velocities from numerical differentiation added  (nvel= -1)
by J.F.Chandler, 1977 May
This is a control routine for interpolating a planet from its
integration tape (or n-body tape)

IDUMM     ignored
JDT       integer value of the coord. time of the desired pt
FRT       fractional part of the coordinate time of the point
NVEL  = 0 positions determined
1 velocities determined
-1 velocities determined from position y-vectors
LCNTL = 0 use results of last call if possible
1 do not use results (must recalculate y vectors)
-1 use old results only if jd.fract is in same tab pt.
output is put in the xp(1-6) vector